Homo sapiens is an african species and the only extant species, so technically all americans are afro-americans and all europeans are afro-europeans.
Banning white people from playing black music or representing black culture (or claiming that this is disrespectful for the black people that created it), is as ludicrous as would be banning black people from using light bulbs, microphones, tv, spanish/electric guitars, pianos, cars... invented by white people, therefore "white culture" (and often built by asians).
What's more, I think restricting "black" culture to black people only, carries the risk of keeping it fringe and preventing it from becoming mainstream.
Now Porgy & Bess is solidly mainstream of course, and black culture is absolutely a vital part of its context and necessary to understand and appreciate it, and there's absolutely something to be said for ensuring that it won't be separated from that context, but I think it's also possible to have it performed by white Hungarians in a way that does justice to that context, and I certainly think it's important that white Hungarians are able to enjoy it.
And then there's also the option of translating it to a different ethnic group in a somewhat comparable situation. Not sure what a good fit would be in Hungary; Roma maybe? Muslim immigrants? That would certainly make it more immediately relevant.
Banning white people from playing black music or representing black culture (or claiming that this is disrespectful for the black people that created it), is as ludicrous as would be banning black people from using light bulbs, microphones, tv, spanish/electric guitars, pianos, cars... invented by white people, therefore "white culture" (and often built by asians).