Slime, Ice and Honey block's outline have a transparent appearance and a thicker width compared to other blocks

Slime blocks don't have a "black" outline like other blocks are supposed to, they somehow have a transparent one, which seems to stick out. This doesn't seem to be the case in "Fabulous" or the line width in "Fabulous" is reduced. I think transparency is intended, but that it's used to stick out so much?