I think of it this way. * fill in - "in" means to fill (write) something "in that space". So it's appropriate for "the blank", which is a location. * fill out - "out" means "completely", so it's appropriate for something more substantial than a single entry, such as a form. So, "fill in the blank" and "fill out the form" are common an natural. "fill in the form" and "fill out the blank" are not, in my opinion.

With respect to an entire form, I think there's a U.S./British usage difference. In the U.S. we definitely say "fill out a form." I think the British say "fill in a form." (Certainly, the Beatles do in the song "When I'm Sixty-Four!") With respect to an individual blank or field in a form, in the U.S. we say "fill in the blank." Yes, truly... to a U.S. speaker it is natural to say "Fill out the form completely. Make sure you fill in every blank." I don't know why.

I second what Kareem has said. Also, I have never heard "fill out the blank" - it's always "fill in the blank."

I don't think there is a major difference but I myself say "fill in the blanks" and "fill out the forms".

both are acceptable and have the same meaning . " fill out " can also mean become fatter