That's because Blastbones is a pet, so technically you aren't the one getting the kill. Blastbones has a lot of bugs/inconsistencies like this:

- It cannot proc sets
- Its damage doesn't show up on the Battlegrounds scoreboard
- Scoring a kill with it will not proc Vicious Death (technically part of the first bullet but worth a mention)
- The disease status effect that it procs bypasses the cooldown of the AoE tick of disease (normally you can only proc the AoE disease every 4 seconds but Blastbones procs it every time because the cooldown is being sourced on each individual Blastbones)
- The disease status it procs cannot be extended by Serpent's Disdain

There's a few other annoying things with it too, but some are pretty minor/niche.

The bug I hate the most about Blastbones is the fact its kills won't count for Soul Harvest...