I've seen that before on a plus I think. I believe it is a glitched out version of the sad Mac. Do you have any other boot floppies you can try? It could be a problem with the system files that prevents a plus from booting but not a 512k.
If you don't put in a boot disk, does it have the disk with blinking question mark on a normal checkerboard background? That info could help distinguish between a hardware and software problem.
edit
Upon re-reading the original post, it seems that it always has this blackness even before inserting a disk. This would point to a hardware problem with the logic board.
Your dead CRT may be fixed easily, a lot of us here have fixed old Macs that were dead by simply resoldering a connector or replacing a capacitor. There are a few likely culprits if you care to try to revive the one that isn't working. If the glass picture tube itself is broken, though, there's little you can do.
If you don't put in a boot disk, does it have the disk with blinking question mark on a normal checkerboard background? That info could help distinguish between a hardware and software problem.
edit
Upon re-reading the original post, it seems that it always has this blackness even before inserting a disk. This would point to a hardware problem with the logic board.
Your dead CRT may be fixed easily, a lot of us here have fixed old Macs that were dead by simply resoldering a connector or replacing a capacitor. There are a few likely culprits if you care to try to revive the one that isn't working. If the glass picture tube itself is broken, though, there's little you can do.