In theory

They're both urethane. There isn't going to be a large difference

Why do you need 2 urethane balls? Why not take a ball with a reactive cover and drill it weak?

Plastic like the Track Spare+ or Storm Mix

Or weaker reactive ball drilled weak. If anything reactive is too much then plastic

SVStar obviously has never thrown both and has no idea how beneficial urethane is. Weak resin is not often the answer.   You end up getting over/under because you can't get into the oil and it still bounces too much off the dry. And it's way better than plastic. 

There is a huge difference between the two and very possible to have both. The Black Urethane is very close to original urethane in the 80s. The cover is very early and is hard to polish.  It really requires oil up front for it to be effective.

The Blue is unique in that it pushes much easier and can be thrown away from the pocket easier.   It definitely is a compliment to the Black and polishes very easy.  I prefer the Blue in my league because I can stay much farther right than the Black.  It sounds like this is the case for your friend.

SVStar obviously has never thrown both and has no idea how beneficial urethane is. Weak resin is not often the answer.   You end up getting over/under because you can't get into the oil and it still bounces too much off the dry. And it's way better than plastic. 

I've thrown the Blue and I have a Pitch Black, don't make assumptions like that

A lot of times people think a highly polished weak reactive ball will work on dry lanes, but they just see over/under. Changing the surface to 500 or 1000 makes a big difference in those situations. From what I have seen, the Black Hammer doesn't look too different from the Pitch Black, maybe a touch earlier