I was in the understanding that artifact abilities counted as a sorcery thus could only be used on your turn.
A player can activate an activated ability even if it isn't their turn (C.R. 117.1b, 117.1, 117.3), unless the ability states otherwise (for example, if it says "Activate this ability only during your turn" or "Activate this ability only any time you could cast a sorcery" [C.R. 307.5]; examples include Humble Defector and Defiant Salvager) (C.R. 602.1b). This is true regardless of whether the ability appears on an artifact or not.
Note, however, that abilities don't count as sorcery spells, instant spells, or spells of any other kind (compare C.R. 113.1, abilities, with C.R. 112.1, spells).
In a two-player game, your opponent attacks you with Ridgescale Tusker with no damage marked on it. Then, your opponent gets priority and passes (C.R. 508.2, 117.3d).
You cast Command the Storm, putting it on the stack and targeting Ridgescale Tusker. Note that the spell doesn't do anything at the moment; rather you get priority. You pass.
Your opponent activates Dragon Blood's ability, putting that ability on the stack above Command the Storm and targeting Ridgescale Tusker. Note that this is an activated ability, which your opponent can activate any time they could cast an instant (C.R. 117.1a-b).
All players pass, then that ability resolves, Your opponent puts a +1/+1 counter on Ridgescale Tusker, so that Ridgescale Tusker is now 6/6. The ability now leaves the stack, so that Command the Storm is now on top of it.
All players pass, then Command the Storm resolves and deals 5 damage to Ridgescale Tusker. This is not enough damage to destroy Ridgescale Tusker, though, since Ridgescale Tusker's toughness is now 6 (C.R. 704.5g). Command the Storm now leaves the stack.
Note, however, that abilities don't count as sorcery spells, instant spells, or spells of any other kind (compare C.R. 113.1, abilities, with C.R. 112.1, spells).
See also this thread and this thread.