Modern popular music would be completely unrecognizable—for one thing, rhythms would be far less complex with syncopation in particular being almost absent.
I agree that music would be wildly different, but the idea that syncopation would somehow be unknown to Europeans is absurd. It's been prevalent in European music since, oh, about the 15th century. Granted, this was not
popular music per se, but I'm pretty sure there's a whole universe between 'folk music' and 'formal music', and at some point, someone is going to fill that void. Probably with music that you can really
dance to.
Personally, I also doubt that rythms would stay overwhelmingly simple, for the same reason. My own view of musical evolution is that you have what's often called 'classical music' (i.e. European formal music), and you have folk music that exist everywhere, and is often pretty simple. "Black music" wasn't more complex than, say, Irish music. Not at the outset. What changed things - in a huge way - was... globalisation? Well, not quite that, yet, but the general tendency of communications improving, literacy spreading, a middle class emerging (a middle class that listened to
music!), and communities in general growing more interwoven and coming closer together. All of the above caused musical cross-pollenation. More and more people being more and more exposed to more and more different influences. People with formal training being more influenced by folk/popular music, various traditions merging, combining, changing each other...
That added the complexity. The idea that "black music" is somehow a unique magical ingredient seems incorrect to me. Without it, we'd see the same general evolution for the same background reasons, but it would just be
different traditions merging and evolving. Popular music would indeed be near-unrecognisable to us, but I doubt it would ultimately be somehow less complex. I bet there would still be songs you could groove to, songs you could sway to, songs that make you cry and songs that get your blood pumping.