It is mostly gray
About one inch long with random shapes and always look good when wet.
Contact the Home Depot
Good application for driveway
27 cubic ft=1 cubic yard of gravel which is 13.5 bags. Your dimensions yield a total of .57 cubic yards of gravel needed at 8" depth. Your project will need 8 bags of the crushed blue stone. Any measurement is never exact in measuring the ground depth. A section of your trench might be +or - the 8" depth. Having said that, add plus 1 bag for a total of 9 to purchase.
Approximately 40 to 50lbs.
This is a crushed Bluestone with approximately 3/4" stones. You should not have an issue with your french drain but if your still concerned then you may want to go with a smaller sized gravel such as pea gravel.
It can be used, but will be difficult to walk on when barefoot.