Magnaflow is nice on 4th gen F-bodies, but the Flowmaster for 4th gens sucks, so you won't see a lot of owners recommending it. Honestly, I'd go as far as to say you'll be happy with whatever you choose. Not often do you see people complain about their exhaust choice after it's installed (especially on a V8.)
Nice details, too. If you ask me, Blackfire produced the best look out of all the sealants I used in the past year (FMJ, Z2/Z5, UPP, and NXT.) It does, however, have the biggest learning curve out of the whole group. What I do is apply it to the entire vehicle lightly by hand and let dry for a little while (nothing extreme, maybe five to ten minutes.) By that point, it's usually good to come off with an easy microfiber swipe. Maybe the colder weather was giving you issues? I noticed that if I don't let it sit long enough or if I tried to wipe on/wipe off, I'd get smearing that was difficult to get rid of via buffing with the MF.
Maybe it just comes down to the whole "different people, different experience" thing that we run into so much on this site. I like the stuff a lot, but it definitely is a bit tricky when compared to something like FMJ, which a monkey could use effectively (you know how easily it goes on/off.)