The darker the square, the more likely it is to contain a block.
Here's the combined heat map for all the Monday to Saturday puzzles.
The whiter corners show that most puzzles start with a 3- or 4-letter word at 1 Across. The fifth row and the fifth column are more likely to start with a block.
The heat map evolves over the week. Monday puzzles tend to have similar grids with few corner blocks. By Thursday, the grid is less predictable, and the themeless Friday and Saturday grids are even more different. Note that center-square blocks are more common in themeless puzzles, which are less likely to have long answers across the middle row.
Sunday puzzles are large enough to accommodate a wide variety of grid patterns, but blocks are still more likely in some locations than in others.
The combined pre-Shortz results have greater contrast between white and black squares, implying less variation in the grid shapes.
Colors are normalized. Pure black here means that location is the least likely to contain a block, not that there's zero probability. Black or white squares can appear anywhere.
You can view the white probability of each square by hovering your mouse cursor over it.