I have a variant of this.
My internal drive died long ago so I have two external drives. I have HS on a SSD formatted as APFS and Sierra on a HDD formatted to HFS+. I am migrating to using the HS install as my default boot, but for the moment I have the two options. If I restart from the SSD everything is fine and I can select which to boot into. However if I am in the Sierra HDD and restart I encounter the black screen with visible and moveable cursor. I've left it for 30 plus minutes before.