I like the +15 health on hit from the Black Box as you can leach health from distance, but the higher clip from the Rocket Launcher saves my life a lot. I haven't been able to reliably identify situational usages for these two weapons on my own.
What are the advantages and disadvantages of the two in different situations? When should I use each?
I am a very rocket-spammy soldier, frequently shooting into big groups of enemies assuming it will hit something. As a result, the black box is well suited to my play style.
The one significant drawback to losing a rocket that hasn't been mentioned is related to rocket jumping. If I jump, I can easily fire two rockets before landing (see earlier: rocket spamming). When I do land and am presumably near combat, I already have an ENTIRELY empty clip. This has gotten me into pretty big problems in the past, and sometimes I do truly miss the regular rocket launcher.
Either way, I think it's a matter of personal preference based on play style. There really isn't a "right" answer, since everyone plays a little differently.
Black Box fits more to the solo-player style. In these case you can roam alone through the map and exploit the +15 HP per hits bonus.
If instead you prefer staying with or lead the "front" attacking group with medic(s), it is better use the standard rocket launcher because before the fight starts you will be already over-healed by your medic(s). In addition, if your medic(s) deploy the über to you, 4 rockets instead of 3 can be a big advantage and lead to better results.
The Black Box does not grant overheal when you score a hit with it, so I'd rather get a Medic to follow me and be able to overheal me than sacrifice one shot at dealing more damage and potentially scoring more kill(s).
Then again it could simply be because I suck at maximizing splash damage...
Using the Black Box is good if you want to attack alone, however if you like staying in crowds, the regular Rocket Launcher is better.
Much like the principle of the Blutsauger, when you want to attack alone, the Black Box can save your life. Using it, you should have your shotgun with you just in case you don't have time to reload. When you are with a group, others will protect and heal you, so you would have time to reload which then would make the black box and shotgun useless, so a Buff Banner, Battalion's Backup or Concheror is more useful to help your entire group and push forward.
The black box has moments where it can be useful, but ultimately it's a very situational weapon; too much so to recommend. When you get down to it, missing one rocket is huge. That can deny you a kill on an overhealed soldier, leaving you where he has 1 to 2 rockets left and that will completely disregard that extra 45 you gained from 3 simple splash damage shots. The black box is only better if you're going to be spamming from afar to deal small damage and chase off enemy targets, but it's horrid at direct DMing when against players who are actually skilled. However, one big downfall that MANY seem to completely overlook is the viewmodel is enormous. Personally I play without viewmodels, but to someone who does it creates such a blind spot that a scout could approach them from their right side completely unnoticed and start batting him to death.
In short, it's too situational to recommend and there are multiple factors (viewmodel, reduced clip for RJing, and just the general lack of necessity for the extra 45) that make it not even worth using.
The black box is very handy to guard a spot that has access to an ammo box but no health pack. As mentioned before, if you've got a medic handy, the regular rocket launcher or direct hit is much more useful.
I personally recommend black box for mvm, because once it's fully upgraded, then there' nothing that can stop you. And as a bonus, if you upgrade soldier abilities, then you can calmly rocket jump while still having an extra 10 rockets and healing yourself like 60 health per hit.
The black box is better because you have a 15+ health bonus in any situation so you are always ready.
