Did you prime the paper with anything to slow down drying time? Or use the colorless blender marker? I do use prismacolor markers for grey toning though and it's possible to get some fairly even, almost airbrushy effects.
I use colorless blenders, and 10 and 20 cold greys among other things. Highlighters, erasers, my finger, a rag, I keep a small brush wet and handy and use pelikan white and even the water. But be really delicate with it and use it mostly only with hot boards or stiffer vellum.And yeah, you have to replace the markers alot, but I keep the dry markers in a handy bin too, for some effects.
Really good vellum is better. The best stuff I've found was some stuff left in storage with a lot of old past-up equipment at a place I used to work. I found two piles of layout boards that were super slick, and had the non-photo-blue printer marks but no layout lines, and I found a box of vellum-like stuff, that was thicker, firmer than the stuff you get at the art supply store, and about a smooth as vellum, so I'm guessing its just really good vellum that they used with ruby and line tape. I only have one ream left, dammit.
Ah, cutting ruby and line tape. Two more skills you tablet guys never needed.
I like to hunt around for new papers to use too, I used to go to this one store out in the boonies, like a craft place that get all sorts of crap from the paper companies. You coulkd find some really great papers for cheap, but usually in loose piles. And often huge cuts like 2'x3'