So that bearing customers don't go crazy, bearing manufacturers have standard sizes. So 6902 and 6903 each specify exact, consistent sizes and types (both of these are what are called "deep groove radial ball" bearings). So any 6902 bearing should physically replace another 6902, as far as the physical size.

There are bearing classes, with a higher number reflecting tighter runout and concentricity. So ABEC-9 has better specs than ABEC-3. As dabac points out, this precision difference may not be an operational concern for a bike wheel. And, as you correctly point out, most bike bearings are ABEC-3, which is a perfectly good grade.

There are tolerance and preload specs. This refers to how "tight" the bearing is at room temperature and allows users to select a bearing that will work properly after its up to speed and warmed up. So you can get C2, C3, and so forth. Most bearings are "CN", which is between C2 (tight) and C3 (looser). You probably don't have to worry about this.

The LBLU does indeed stand for seals: one "LB" and one "LU". This is not super-common terminology, but I suspect it refers to what is more often labeled LLB and LLU. An LLB seal is a metal-backed rubber seal that is fastened to the outer race, and does not contact the inner race. An LLU seal is also metal-backed rubber but is fastened to the outer race and contacts the inner race. The former gives lower friction but lower sealing integrity, the latter has more friction and better sealing. If the picture you show really is your bearing, the sealing didn't work too well. That's a pretty rough bearing.

Enduro sells bearings for bikes, and they use a slightly different terminology. Their LLB does make contact, but to a lesser degree than their LLU. Enduro LLB and LLU seals have grooves machined into the inner race for this inner seal. Standard metal-backed rubber seals (RS type) just contact the bearing surface with no groove. I use Enduro LLB bearings, but that said I think that you probably would be fine with inner and outer RS type seals. That is, I use 6902 or 6903 LLB but I think that two RS type seals (2RS suffix) would work. More info here.

Historically, bearings were loose inside an angular contact cup and cone setup. One would maintain wheel bearings by taking them apart and cleaning off dirt and old grease in the cup and cone (and possibly the balls) and then repack the bearings with new grease (and possibly with new balls). You can pull the seals off of some cartridge bearings and can then clean them using solvent of some sort, and then repack them. Given your picture, your bearings are shot and this is not an option. And part of the whole point of cartridge bearings is that you just replace them, with less work than cleaning and repacking. But you can buy bearings with a snap ring (SRS) seals that are easier to pull out, which allows you to remove the seal and clean the bearing. You can also remove LLU/LLB and RS bearings, but its tedious and you can damage the seals so that they don't seal as well when you replace them.

Upshot? Any 6902 bearing should replace another 6902. Same for 6903. You could buy a bearing with LBLU seals, but these didn't work very well for you, judging by your picture. You'd probably be find with 2RS bearings seals too. I value my time, and so I pay a few bucks extra in premium to get a bearing that I'm confident will have proper design for bikes, at Enduro. These are LLB seals.

If you've been riding a bike with the bearing shown, anything you buy will be a sea-change improvement. I suspect that you do not need to shell out for ABEC-9 or ceramic bearings! But of course its your call.