I don't think that the Black Box is all that great. 15 HP seems nice on paper, but that's less HP than a medic heals in a single second. The HP bonus does stack when you hit multiple people with splash, but IMO it's not worth the decrease in clip size. Three rockets isn't very much and they can disappear a lot faster than you'd like, especially if you like to play aggressively as a soldier. It's pretty common for me to do things like rocket jump twice to chase down an enemy medic and then use the two remaining rockets to kill the medic. With the Black Box, you're left with only one rocket after the second RJ, and if you miss with the last rocket, you've just propelled yourself deep into enemy territory with no ammo loaded. Pocket soldiers don't need the Black Box because of all the heals they should be getting from their medic, and roaming soldiers probably won't like it because they suffer too much from the decreased clip size.
I do like the Battalion's Backup. It's a lot better than the Buff Banner and it's good for all of the reasons that the Buff Banner wasn't. The Buff Banner charges by hurting enemies (and it requires a substantial amount of damage output to charge), so it's really a weapon that only helps when you're already winning. The Battalion's Backup is the exact opposite: it charges when you take damage, so it's more useful for trying to reverse situations where you are at a disadvantage. It's also an effective Kritzkrieg counter, and it can be a real help for mid fights when the enemy team gets Kritzkrieg before your team has Uber.