Well,this is, undisputably, what happened IOTL. But I don't think it's that hard to imagine a TL when things go slighltly different and a united Moroccan kindom colonizes the Canary islands and eventualy, the Caribbean by the 1300s/1400s.
We just need to find a reason for doing so (and also slightly improved vessels).
Reasons such us:
1) Fishing
You're gonna have to come up with a better reason than that - when islands so close to the coast of a continent remain virtually untouched by the local seafaring Muslims for
that long (we
are talking about a period of at least 700 to 800 years here), then there's bound to be a pretty good reason for that.
It propably has something to do with the fact that (IIRC) the waters around the Canary Islands are subject to some pretty strong sea currents that make it awfully hard to get from the islands to the Moroccan coast.
And IIRC (though I could be mistaken in this), the main reason the Spanish settled the Canary Islands in OTL, was that they needed supply depots for ships going to Africa or the Americas, and Canary Islands provided a proper alternative for bases on the Moroccan coast, which was rife with pirates and usually hostile local states and tribes.
The Maghrebi Muslims, on the other hand, simply don't have a reason to avoid the Moroccan coast, as they're the ones who control it.
2) An easier way to get to West African gold (let's say, for example, a Berber sect of Shia Muslims forms a kindom in Mauritania that is hostile to sunni Morocco - or viceversa - and blocks trans-Saharan trade routes). The Canary Islands became a base for these ships. Eventualy, a ship is blown towards the West by a storm and comes back with stories of excellent land, friendly people and easy available gold (I know this wasn't true, but, if Columbus exagerated, why wouldn't his Muslim counterparts do the same). Colonization starts soon after this.
Not plausible - a situation much like that existed in OTL: Shi'ism was traditionally very strong in the area of
northern Morocco and northern Algeria (Fez was, in fact, founded by the Shi'ite Idrisids), and Isma'ili Shi'ism remained popular in the region until it was rooted out by the Almoravids, who arose in southern Morocco only after the local (IIRC only recently Islamized) Berber tribes adopted the Sunni doctrine after contact with Sunni scholars from Kairouan and the Abbasid Caliphate.
And yet, the conflicts between the Shi'a and the Sunnis in the Maghreb never led to either party blocking the trade routes to West Africa.
What's more; in OTL, the only Moroccan states that were truly powerful enough to control (and thus block) the West African trade routes, were
also powerful enough to subjugate the northern Berbers and even conquer Islamic Spain, in spite of the religious differences.
In other words: a southern Moroccan Muslim state powerful enough to block off the trade routes would have to be so powerful, that it could afford to go much further than just blocking off traderoutes - such a state could (and most propably would) take over the entire Maghreb, just like the OTL Almoravids and Almohads.
And on the other hand, I really don't think that a state
isn't capable of
conquering a rival Muslim state in the north, has what it takes to completely block off the trans-Saharan trade routes.
And lastly; for a state like that, blocking off the trans-Saharan trade routes is likely to hurt
its own economy more than anyone else's. At least during the Medieval Ages, the trans-Saharan trade was a very important part of the Moroccan economy.
Also keep in mind that, should the trade routes through Morocco and Mauretania be blocked off, it is more likely that the trans-Saharan trade routes would shift
eastwards (IIRC there already was an important caravan route from Kairouan to various cities in the Sahel IOTL, and the richest Sahel states were inland states that were easy to reach by caravan), and that nomads like the Tuareg would come to play an (even more) important role in it, as the nomads of the Sahara won't be under the control of whatever state is calling the shots in southern Morocco.
3) The Cruzaders/the Bizantines/the Mongols manage to get control of Egypt, thus blocking the trade routes between Muslmim North Africa and Persia/India.
A complete, long-therm Crusader conquest of Egypt would be very hard, if not impossible to pull off, and the only realistic way to have the Byzantines control Egypt, is by not letting it fall to the Muslims in the first place.
As for a Mongol conquest of Egypt; that'd be a tad more plausible than a Crusader or Byzantine conquest of Egypt, but that's about it.
..
IMHO, the only non-Muslim nation that ever had half a chance at conquering
and holding on to Egypt during the Medieval Ages, was Makuria.
However, you'll need a pretty early POD in order to get a plausible, long-therm Makurian conquest of Egypt - the best POD for this would be somewhere during the 740's, when the Umayyads were collapsing and when there were a few Coptic revolts in Egypt (IIRC there were seven or eight major Coptic revolts in Egypt between 700 and 850, but I don't know wether there was a major revolt during the 740's).
But in this scenario there's the problem that, if the Makurian's OTL attitude to Muslim merciants is anything to go by, it's
very unlikely that the Makurians would block the trade routes between Egypt and the Middle East, so even though you'll get a very interesting scenario in which the Muslim world is divided into a western and an eastern half, you propably won't get a situation in which desperate Muslim merciants try to round Africa.
Somebody in Moroco tries to emulate what the Carthagians did, his ship gets blown to America and voilà, you have a Muslim America.
...that's
got to be one of the worst oversimplifications I've seen on this site.
Suppose the crew doesn't get shipwrecked,
suppose they don't die from thirst, hunger, disease, scurvy, or starvation,
suppose they're not killed by hostile natives, and
suppose they want to return at all ("we drifted thousands of miles over unknown seas, we finally found an island - and a suprisingly hospitable island to boot - and now you want risk our lives
again by going
all the way back!?"), even then it's far from likely that we'd see an Islamic America as a long-therm result.
If they, like Columbus, just end up in the Caribbean, then they'll encounter nothing but a few tropical islands with weird-looking natives that still live in the stone age - not exactly something that's worth crossing the ocean for.
And even
if they encounter one of the richer Amerindian civilisations and bring home some curious jewelry (which would be a best-case scenario), then it's
still not likely that the local ruler would send out his ships to the New World or that that would have any lasting results, because hey, why bother risking a lot of resources and manpower on crossing the Atlantic and establishing colonies on the other side, if you can gain even greater riches from empires (such as the Songhai Empire) that are practically right next door?
...which ties in with tallwingedgoat's point; the Muslim nations (and Morocco in particular) simply have much more room to expand in the Old World. Couple that with the fact that Morocco had far less resources and manpower than Spain, and you have a few very good reasons for Muslim nations like Morocco to be uninterested in the Americas, and focus on more realistic goals and targets.
I'm sure somebody could come Pod. But I don't think that a Muslim America is so hard to get.
Sorry, but I really to beg to differ.
I'm not saying that an Islamic America is impossible, but in order to realize such a thing, you'll need a POD that's
much more radical than a few Moroccan fishermen that accidentally cross the Atlantic.
I'd say that you'll
at least need to maintain a strong Islamic Spain, and you'll also need to take the potential naval powers in Christian western Europe out of the equation somehow.