A black screen when starting FS-UAE could be due to missing kickstart. See http://fs-uae.net/kickstarts for information about what ROM files FS-UAE expects. Regarding 100% and buffer underruns: You will get audio buffer underruns if the emulation cannot run at 100%. Too low host performance is also why your CPU is at 100%….
You can see http://fs-uae.net/performance for some tips on how to reduce CPU usage (lowering accuracy, etc). The Raspberry Pi is probably too slow to run FS-UAE at 100% speed with any settings though.