that's how safestrap works. Safemode in safestrap boots into whatever custom rom you install, if none is present it doesn't know what to do so you get a black screen.
Turning off safemode/disabling safestrap loads your stock rom.
So the terminology may be unfamiliar. Safemode= boot into custom rom while your stock rom is "safe", not safe as in you are safe because you're back to the stock rom. Safemode off/disabling safestrap boots the stock rom.
In this type of system it seems like a backup is unecessary since booting into a custom rom keeps your stock rom safe and to get back to stock you have to be out of safemode or have safestrap disabled.
Please correct me if I'm wrong but that seems to be my understanding.