We're really splitting hairs here though.
For the purposes of the game, I don't think it is unreasonable that the blade mods could be compatible with the pickaxe.
I mean sure, technically the shovel head is a blade, but who thinks of it as such? Would one really pu a tempered or serrated edge on a shovel?
Also, how come we can't have both on the same tool?
Now if you google shovel head you'll get much more interesting stuff by the way.