Oh yeah, with a very small axolotl, sand is ill-advised but only for a specific reason. How do you feed your axolotl? Baby ones are usually hand fed, or directly fed. Like, I know people will take their baby out, put it in a tub, and feed it there, with zero sand, so the axolotl can suck up the worms, the pellets, whatever, without any fear of sand. Sand, even at its finest, is a threat because the little axolotl is so small, it might choke on the sand or get impacted. Once they're like 10 inches long, sand rarely causes an impaction.
Like those aquarium marbles, you know what I mean? Those might as well be marketed "Aquarium stones! Guaranteed to kill axolotls!" Cuz they just suck them up when they go to suck up food. I got my little girl a dish and sank it into her sand, and I feed her directly, she comes up to the surface, but if she misses, I use my turkey baster to suck the pellet up, sand and all, put it on the plate, and just suck up the pellet without the sand and try again. Some people feed theirs exclusively from a dish like that.
This is just personal experience, I am no expert. Also, and this is just me, my girl now has brown sand and it's really hard to see if she uses the bathroom, so idk if black sand would be any easier to see. I miss her white sand, made seeing things easier.
And hey, I'm real sorry about your axolotl. I hope this experience doesn't turn you away from them. They're precious creatures that we as a people have dwindled to a handful. It is an honor to try and raise one to make up for what our ancestors did to them. If you're thinking on trying again, keep the tank going, don't change the water for a bit, let the filter keep running, for like a month, then change the water, test it, etc, etc. I had a spare tank sitting around that I used as a dummy tank, that I pre-cycled some filters in. Don't be shy to ask for advice or whatever. If no one replies, you can always try to message me directly. I float around.