The term "blank canvas" can refer to a number of different things, both literal and metaphorical. However, all meanings have in common the idea of an empty starting point.
Literal
The literal definition of a blank canvas refers to a white or off-white piece of cloth. A blank canvas for artists to paint on is often stretched tightly over a wood frame and stapled to the inside of the frame.
Other Art Definitions
A blank canvas can refer to artistic supplies other than canvas that have not yet been filled with art. For example, a piece of paper could be considered a blank canvas for a writer or poet and a roll of photographic film is a blank canvas for a photographer.
Metaphors
The definition of a blank canvas can include metaphors that have nothing to do with art. For example, somebody just getting into the world of politics may be called a "blank canvas" because he does not know much yet about working in that environment.