Why does the GB have H-blank & V-blank periods in the video timing ?

The terms H-blank & V-blank are a part of Cathode Ray Tube (CRT) based video games and as a result they have carried over to some LCD-based games. The H-blank period is the amount of time it takes the scanning electron beam to return to the left part of the screen and the V-blank period is the amount of time it takes the scanning electron beam to return to the top of the screen for CRT displays.

Since the Liquid Crystal Display (LCD) of the GB doesn't have an electron beam there are no lag times that require an H-blank or a V-blank period. Since the GB doesn't have dual-port video RAM, allocating these times anyway provides a convenient time for writes to video RAM since the video circuits aren't updating the screen during these periods (and as such aren't preventing access to video RAM during these periods.)