In an attempt to cure this problem I created an image of the board with quadrant lines drawn in. Then I made several randomized lists of the 64 board coordinates, from a1 to h8. The drill is that I go through a random list from beginning to end, pointing out the square of each coordinate on the board.List 1: c5 h2 e5 c3 a5 e1 c7 b6 d8 g3 e7 e3 a8 f4 f5 b5 a3 e4 e2 d4 g8 f8 a7 g1 a6 g7 g4 h5 b7 a2 d1 d2 b4 d5 c1 h1 b8 h4 c4 b3 h3 f6 e8 a4 b1 d7 d3 f2 g6 h7 g5 a1 g2 c2 f7 c8 h8 e6 d6 h6 b2 c6 f3 f1
List 2: e4 f6 b2 e3 g4 b7 a3 h3 c1 g6 e8 c8 a5 c7 c6 e6 d1 b6 g7 d7 d2 b5 c5 h5 a2 d8 g8 g2 b3 h2 h7 e2 c2 h6 b1 a6 a1 a7 f4 b8 h1 f8 c4 e1 g3 g5 h8 e7 h4 f1 c3 e5 d6 f2 a4 g1 a8 f7 f3 d3 b4 d4 f5 d5
List 3: e6 d8 d4 f2 e1 g3 h1 a6 b2 c2 e3 b6 g8 f7 h6 c5 a2 a4 d6 g5 h3 e8 h5 f6 d2 h2 b1 c6 e4 g1 c1 f1 a3 f5 g2 c7 a7 c3 f4 d7 a1 b4 h4 b3 e2 b8 a5 f8 a8 h7 b7 c4 f3 g6 e7 h8 g4 g7 e5 d1 c8 b5 d3 d5
Next I do the same with a 4x4 board, going through the list again, pointing out where each board coordinate is inside its quadrant.
List 4: g1 a3 f3 g6 d4 e3 c2 h8 b1 d6 d7 h2 b3 b4 g5 g3 a4 h6 a1 b7 e4 f7 f2 a2 e2 c8 h5 d2 c3 d5 d1 g2 f5 e8 a7 e7 h1 f6 b6 g8 b2 c6 a8 h3 f8 g7 b5 f1 a5 d8 c7 a6 h7 c5 d3 c4 b8 c1 h4 g4 e6 f4 e5 e1List 5: e8 d5 b5 a3 e2 b1 g2 g5 h7 c8 c6 g6 h4 a7 d3 g3 a8 h1 d2 a4 f2 d7 e1 c2 d4 g8 b6 c4 a6 g7 a1 a5 g1 h3 e3 f3 f4 b2 c1 e7 b4 d8 c3 d6 b3 h2 h5 b7 f1 f5 h6 e5 g4 f6 d1 f8 c7 e6 a2 c5 e4 b8 h8 f7
List 6: d8 c5 f7 c8 d6 h1 e3 f2 d2 a1 e8 g6 a7 c4 c6 g5 e5 c1 h5 b5 h3 h8 f3 g8 a2 a3 c7 h6 e6 h7 g1 b2 g3 e4 h2 g4 f6 b4 e2 a5 h4 c2 f4 b3 b6 a8 d7 g2 f8 c3 f5 d1 d3 g7 e7 b8 a6 a4 d4 e1 b1 b7 d5 f1
After doing these drills I visualize the board not as 64 squares, but as 4 quadrants with 16 squares each. The quadrant lines have become points of reference that has made it easier to visualize the center and orient myself around the board. When evaluating blindfol if two distant squares are on a diagonal, I fist visualize where the first square is inside its quadrant, and then where the second is inside its. With this information it has become possible to know whether they are on a diagonal or not.