All the transgender men of colour are gay. Autogynaephilia is only present in Western cultures. It's a middle-class phenomenon. Most POC are not middle-class.

Here's something that I suspect may be a contributing factor, at least in the U.S.:

Most "transbians" are AGP straight white dudes. Their AGP usually seems to revolve around a "sissy/humiliation" fetish: being humiliated turns them on, and since they view women as inferiority personified, casting themselves as "women"-- i.e., "sissification" (= feminized in an ultra-weak, degraded, porn-y way) -- is the essence of hawt. It's exciting precisely because it's a transgression (if you will) against their own "superior" status as straight, white men... forbidden fruit. Same principle as the straight white CEO who gets a kinky thrill out of groveling at a dominatrix's feet. So naughty to be powerless!... but of course, in reality, he still calls all the shots.

By contrast, black men are ALREADY deemed inferior in mainstream American society's eyes. There's nothing forbidden, or transgressive, or daring (much less kinky/hawt) about it; this is just shitty business as usual for them. So I doubt that the straight ones are gonna see much appeal in a "sissy/humiliation" fetish. Which means that few, if any, will ever become "transbians".

Maybe it's not so much that gay black men are especially likely to "go trans"; rather, it's that straight black men so rarely do. Because what's escapist fun to straight white men represents the very opposite of that to them.