Your upload speed doesn't matter, as far as why it's a bad idea.
One, the Twitch ingests tend to start screwing up past 3500kbps, which isn't enough to deliver 1080@60 with any kind of decent quality. Barely enough for an OK-enough 1080@30. Your viewers will also start complaining about buffering problems if they're on a moderate to weak connection (after all, they need to download what you're sending) even at 3500, and if you risk the ingests screwing up and go even higher, then your viewer base will shrink further. Then, there's the fact that the Flash player SUCKS, and can only actually decode around 30-40fps at 1080p even on a top-end system, even if everything else is met.
Again, unlikely to be related to your blackscreen issue, entirely tangential, but also still a pointless exercise that you might want to drop, and improve your delivered quality by a noticeable amount once the blackscreen gets fixed. No one's going to come by just because you toss 1080@60 in your title.
They'll just leave if they end up in buffering hell, or if it looks like pixellated crap due to insufficient bitrate.
