Mayy just be me, but I'd love it if Crest of the Rider was added to the slot enchant dude so I could waste all of my money trying to slot/refine both those and my Black leather boots~
Honestly, I understand that the only way to obtain slotted BLB's are through slot attempts, so it'd stand to reason you should be able to get the other part of the combo the same way. But not so, and I've never understood why.
Even though Crest of the Riders are available with slots, it's horrendously rare (card rate drop from a miniboss), and you shouldn't ever expect to get it. The non-slot ones however are readily available by slaughtering the halfway innocent denizens of Skellington, but are relatively worthless due to the fact they can't be slotted.