I've purchased a few blocking boards over the years, but the best one I ever had was one I made out of white PVC sheeting, aka Sintra brand. Its smooth surface will not snag, it is non-hygroscopic (nonporous), durable, and you can easily insert push-pins into the soft plastic board.
I bought a piece of the 1/2" thick, white, PVC sheeting from a home improvement store (Home Depot, Menard's, or maybe it was Lowe's), cut it to 30" square (but you can make any size you like), and sanded the edges. Then, using my Fletcher manual matcutter's straight blade on both sides of the board, I scored very shallow, perpendicular lines to make a grid of 1/2" squares. As a guide for the scoring, I made very small pencil marks along all four edges of the board, and erased them later.
For the blocking, I suggest following the instructions given in the PPFA Guidelines for Framing Textiles and Needlework, but here's a brief description:
Along the scored, perpendicular lines of the board, align two edges of the moistened (not soaked) needlepoint and hold them in position using push-pins. Then, pull the other two sides into squareness under moderate tension and use more push-pins to secure all sides of the needlepoint. The blocking process may require two or three stretchings in order to square it completely.