To find out how many slices of the blueberry pie they ate, we first need to determine how many slices of the apple pie they consumed.
The apple pie is cut into 6 equal slices, and they ate \( \frac{5}{6} \) of it:
\[ \text{Slices of apple pie eaten} = \frac{5}{6} \times 6 = 5 \text{ slices} \]
Since they ate the same fraction of the blueberry pie, we calculate how many slices of the blueberry pie they ate. The blueberry pie is cut into 12 equal slices, and they ate \( \frac{5}{6} \) of it:
\[ \text{Slices of blueberry pie eaten} = \frac{5}{6} \times 12 = 10 \text{ slices} \]
Now, to find the total number of slices of both pies that they ate, we add the slices from both pies:
\[ \text{Total slices eaten} = \text{Slices of apple pie eaten} + \text{Slices of blueberry pie eaten} = 5 + 10 = 15 \text{ slices} \]
Thus, the total number of slices of the pie they ate is 15 slices.