Even if you go back many times, the chances are that most places will not offer blacks while almost everyone will have blues. If for no other reason than that, I would take advantage of a black if you have the opportunity. A blue will likely be easy enough to get on a later trip. Yes, there WILL be a later trip.... I do also agree that the blacks are a more interesting animal in both looks and behavior.
This mirrors my thinking exactly, if I could only take one on my upcoming trip I would prioritize the black. A cull black is actually included in my package but I will likely upgrade and I plan on trying to take either a cull or trophy blue to go with it. For me also, the relative rarity of black wildebeest mounts(either shoulder or euro) makes for a more interesting trophy.