The easiest place to farm this is to max your Garrison’s mine to level 3. There are two node types, the regular Blackrock Deposit and Rich Blackrock Deposit.
For farming in the world, it can primarily be found in Nagrand (Draenor), Shadowmoon Valley (Draenor) and Shadowmoon Valley.
Do be aware that the value of Blackrock ore is quite low, and it might be cheaper to simply buy it on the auction house.