I started at that exact same screen for 15 years

yes there should be a knob on it somewhere to make it black again, altho it might be inside, if memory serves me correct there were a couple knobs on the back available via screwdriver

also that monitor was edge to edge in its picture, so it probably needs adjustment, which would require taking off the back, which i would not suggest as it can kick you about a foot back if your not careful and something like that would almost require the monitor to be live while doing the adjustment

ask around, theres usually an old tv repairman somewhere willing to help a kid out