You are going to be trimming off excess with either width tape. If 3/4 is all you need than get that, if you do not know then 1 inch will work no matter which of the two sizes u need.
If you end up with high spots where the window is farther away from the wall in places (maybe the wall is not perfectly flat) then you might need to shove some extra butyl in that space, but before you do this DO loosely tighten the frame as the window frame might help flatten things out.
Tighten some, then a day or two later tighten a little more to give the rubber time to ooze into crevaces at its own speed.
Black streaks? I do not know, coulbe be the pain on the window frame too. You cannot get white rubber, that would be called silicone (what they make white tires out of).