It’s not impossible, but we’d have no way of knowing because the result would be just another black hole. What I can say with some confidence is that unless some wildly unexpected physics emerges, there’s no way antimatter only occurred in dense clumps and collapsed before it could annihilate. So could there have been a freak occurrence yielding a black hole from a dense cloud of antimatter? Unlikely, yet possible, but it would still do nothing to explain why matter ended up dominating antimatter.