a chalk board
Blackboard
A chalkboard is black when clean and dirty when written on with white chalk.
The answer to the riddle "What is black when it's clean and white when it's dirty?" is a chalkboard. When a chalkboard is clean, it appears black; however, when you write on it with white chalk, it becomes "dirty" and takes on a white appearance.
A blackboard seems to fit.
A blackboard
Blackboard
A blackboard.
no but black ink does
A chalkboard ~_~
Black will look the cleanest, white will be the easiest to tell when its dirty, but all 3 are about the same in degree of difficulty to clean which it is fairly easy to clean a microwave.
A chalkboard ~_~
It's mold. You can clean it with bleach.