Perhaps so. And it would have been for the best if Portugal could have retained it's overseas territories AS overseas territories and integrated them into a democratic Greater Portugal on a one man one vote basis. It would have avoided three ruinous civil wars in Angola, Mozambique and East Timor, for one thing, that killed many people, and set all three territories as well as Guinea Bissau on a path to economic growth of which only Angola currently is on. Metropolitan Portugal, too would be both growing and solvent, with Lisbon and Douro being predominantly African cities and huge and growing populations from African migration.
An interesting picture indeed.
But because Portugal chose such a radically different path from the other EU members, Portugal would probably not be permitted to be part of the EU. The EU wants decolonization, not integration of former colonies into the metropolitan nation. And the EU finds any member nation with a rapidly growing population potentially destabilizing.
Well, they'd be in an even more awkward position there than with Turkey in OTL. The cultural and religious links are there and there'll be the awkwardness that many will be saying it's "because they're black." I mean French Guyana and Caledonia are pictured on the Euro, for heaven's sake. Not to mention Greenland.
If it went that way, membership would be blocked probably on the grounds of human rights issues of some sort. They'll be safe preconditions because these African and Asian societies won't come close to European standards for the foreseeable future. If they start to get there the goalposts can be shifted.
Does the EU actually have policy on that, though? Avoiding high growth populations? I'm not familiar with anything of that sort. I do know that
slow population growth is a long-term crisis for many member states.
I'd guess Portugal to still have some chance of entering the EU, if certainly a smaller one.