Anyway, this is a recurring blackarch issue, not Garuda stuff.
Blackarch is not supported anymore in Garuda since a lot of time, since it is supposed to be aimed to skilled users, and we cannot help for that extrrnal stuff.
You could try the mighty
update remote keyring
But it might not work, if there are underlying blackarch issues.
So the best thing would be to remove blackarch from your pacman.conf
If you really need it, you could follow-up this issue in the blackarch GitHub site. There are approx 25 issues there for this same problem. See e.g.