What is sheetrock? What is blockfiller?

The blockfiller is a primer made for new sheetrock. Sheetrock is a board made of gypsum and normally coated on both sides with paper. Drywall comes in many thickness usually 3/8, 1/2, 5/8 and 3/4 in. in lengths from 8' on up.

See Bennette Paint | Primers: Acrylic Blockfiller Masonry Coating 5 Gallon

You can't always believe the first answer you find here. Block filler is just that, for on masonry walls to fill the pores before painting.

Sheet rock is another name for dry wall, also known as gypsum board. I wouldn't be surprised it it is somebody's trade name.