Another user reported disabling nVidia ShadowPlay (Also Overlay function) helped with a black screen issue, maybe that is the cause?
Can you upload a logfile with the current settings after a few minutes' worth of streaming please (The one from the black screen attempt would be handy as well)
Standard internet speed tests are not indicative of actual throughput for streaming performance/potential. Can you go to the below link and download r1ch's TwitchTest utility, it will assist you in finding the best Twitch ingest server for your location:
https://r1ch.net/projects/twitchtest
It is best to do a medium length test duration.
Once it has completed choose the server that as first priority, has the highest Quality and second priority, the lowest RTT (Round Trip Time) For good stream throughput, quality should be 80+.
TwitchTest utility will also provide the estimated potential (Twitch only allows up to 6,000 kb/s) bitrate you can stream to for each particular server as well, which may or may not assist you in regards to your upload speed.
If you are streaming via YouTube, unfortunately there is no such utility available.