6. (10 points) Place the " $b$ " shaped polyominoes on an $8 \times 8$ grid: The polyominoes must align with the grid lines, and after placement, the total number of polyominoes in each row and column must be equal. Then, the total number of such " $b$ " shaped polyominoes that can be placed is $\qquad$ - (Note: The polyominoes can be rotated, but not flipped).
【Answer】Solution: According to the analysis, as shown in the figure,
to make the blocks align with the grid lines, and after covering, the total number of blocks in each row and column of the grid must be equal, 7 more $b$-type blocks can be placed.

Therefore, the answer is: 7.