A blackout is when the power totally fails. The damage that a blackout causes to your system depends a great deal on its timing. If the system is idle when the power goes out, probably nothing will be wrong with the system when the power comes back on. However, if the power fails whilst writing data to a disk you're likely to have a problem. Also the power doesn't fail cleanly, but with spikes and jitters both when it ends and when it comes back on. Some systems survive the power going off and back on without too much difficulty, but the potential for large amounts of damage is there; and of course, you lose any unsaved work.
What is a Blackout?