RAW, I don't see why you can't blind something with Truesight, but I feel like it shouldn't be possible. (Maybe vestiges of previous editions?) I wonder whether I've overlooked some basic rule.
Yes, you can blind something with Truesight.
The only thing Truesight does in this case is magnify the ability that was lost to blindness:
Truesight enhances one's ability to see: through magical darkness, invisibility, illusions, shapechanges, and into the Ethereal (PHB p.185).
But the blindness condition now says "you can't see. The sight through illusions, darkness, &c. which you so enjoyed just a moment ago, you don't have anymore (PHB p.290, paraphrased)."
In your specific example of the Demilich, note that blindness is not one of the (many) condition immunities listed in its stat block (MM p.48).