Screw attack blocks
They use crumble blocks with BTS of $08
The tile displayed is $BF in the CRE (right below super missile blocks).
I didn't include any graphics so you can make the tile look however you want.

I went ahead and did the bomb reactions for them so they display correctly when you bomb them. They use the same collision routine for bomb detection as speedbooster blocks, so if powerbombs reveal those, they "should" work on these as well. No guarantees though on that.