I would look to "blind" monsters from outside of DND before designing elite Grimlocks. -slapping extra Hp, AC and Saving Throws on them is easy enough. One could just reskin the Orc.
But, have you ever watched Bleach? There is a blind guy in that series that traps his opponents in a globe of "blindness" where all their sense, except pain, is muffled. It would be a really unique challenge for the PC's to navigate and kill this guy. Could be their "shaman" or whatever that has this ability. Here is a clip:
https://www.youtube.com/watch?v=NLlXpY5Mm5I
Encounters with grimlocks should be a place where they can utilize their blindsense to their advantage. A burning bulding with tonnes of smoke and noise from the flames, where every step on the floor planks can be felt by the grimlocks.
Or maybe in a waterfall, the PC's might not be able to see a hand in front of them, but the grimlocks can feel the vibration of the characters in the water.
How about the cave of mirrors? Huge crystals reflects perfect images of the PC's which disorients them. The grimlock are not affected though. Which image is a real grimlock and not a crystal?
Everything in darkness is fine of course, but you have you regular dispel magic or daylight to fix that. Get the PC's to think out of their comfort box. How do you dispel smoke? Stop a waterfall? Or clear an entire cave of mirror crystals?
Just some ideas.
