I haven't actually found documentation on this, but it appears that grey represents items that were included in the selected revision but were above the directory that you selected to view the history for.
For example, your project consists of the following structure:
Root
- dir_a
file_a.txt
- dir_b
- dir_c
file_c.txt
If you click View History for dir_b, then a change to file_c.txt
will appear in blue, but a change to file_a.txt
will appear in grey (since it is above dir_b). Clicking View History for dir_a will show both in blue.