I found a couple things that worked, but the one that was perfect was lemon pledge furniture polish. I heard this at rat a few time and it not onl;y made the black cases look new but works on everything else on the bike. The big bonus is that it leaves a super slick finish and dirt just wipes off even with a dry towel. Aside from the brake dust on my alloy front wheel, theres some kind of greasy splatterings on them after every ride that doesn't come off that easy. After pledge it wiped right off with a single wipe and with no cleaning agent, just a dry towel ! Try it, you may be amazed. It's not much of a cleaner as far as getting greasy stuff off, but it's amazing as a protectant and will clean off light dirt, and it makes the black cases look new.
However, my cases aren't old and haggard, so if it doesn't work good enough for yours because they're too bad, i found lexol leather conditioner works wonders on any textured surface to make it look new. I was using that on my cases before the pledge but switched because pledge works on the entire bike so i just use it for everything.
