It would be someone a fair bit older than him. I took five decades to mean that's how long they've been in the industry. Danny Zuko is only fifty something himself. I doubt he would have been in the closet since primary school (or whatever the American equivalent is)
5 decades doesn't mean 50 years. The decade we're in counts as one, then the 00's, 90's, 80s and Danny Zuko started in the 70's - that could count as 5 decades, loosely speaking.
Having said that, I don't think the blind is referring to him - it's too boringly obvious if it is. He hasn't managed to keep it a secret - he's tried, but the whole world and their goldfish have heard the rumours.
I agree that it's someone with no or few rumours so far so it would rule Copacabana out too - does Magnum (as suggested earlier) have any?