African arowanas get big, but are bottom feeders, preferring worms and shrimp to monkeys in trees. I don't know that it would even be considered competition. I wouldn't recommend the black just because as they age they look just like a silver. Why pay double for something that last only a few months. I think the dat is a much better choice.
There are a number of big differences and justifications for a higher price with Black Arowana.
For starters, Silver Aros are almost always mass farmed, and result in being cheap fish considering the size they will attain.
So paying double that price for a fish that is most likely wild caught and of good genes already seems like a no brainer.
Now add to that the fact that a Silver will pretty much garuantee to develop drop eye as it grows, whereas a Black will not.
Silvers are also more likely to develop underbite.
Then add to that the size difference. A Black will make a much more suitable fish for home aquaria being that it grows smaller than a Silver. (Though mine is over 2.5ft and growing)
As for them looking the same as a Silver, that may be the case to an untrained eye.
But there are clear differences such as fin coloration, body sheen and tone, body shape and bulk, head shape and fin size/shape.
for the op, here is some pics of mine:
My Black at just over 2.5ft:
Here is where I decided to try it with my MUCH SMALLER wild Silver (You can just see her tail) note the damage to my Blacks barbels and fins, and bruising on the body... The Silver kicked it's #17#####. Had to be separated within days.
In the end she has lived a happy stress free life with a bunch of monster sized Bichirs, Gar and large catfish.