Are you hoping to use this tarp for all seasons and all conditions? If so, I recommend with with the blackbird you go with a minimum of 10' x 8'. However, 10' for some reason is pushing it with the blackbird after you factor in 101" ridgeline, 4" worth of excess gathered ends, and let's say 4" the natural sag or catenary slope a ridge line usually has, it leaves you with little room on the ridge.
With that said, I'd say 10.5" x 8.5" minimum for 4 season use. For 3 season use I like a 7' width and would like to try 6'.