The phrase you are looking for is plain paper. This means unlined paper. (It may or may not be blank.)
From the Cambridge dictionary:
› paper that has no lines on it: a letter written on plain paper
As for blank paper, the free dictionary lists blank as:
1.a. Devoid of writing, images, or marks: a blank page; a blank screen.
which could imply (I'm not convinced it is 100% clear, but you could make the case for it) that lined paper that has not been written on is not blank, because of the lines.
However, from experience as a native speaker, I have found "blank paper" to be ambiguous: in practice, some people use it to mean completely blank and others use it to mean not yet written on.
For that reason, I would suggest using plain paper for a completely empty sheet and perhaps blank lined paper or unused lined paper to indicate that you want paper that has lines but hasn't been used. (Or you could just say lined paper - people will probably assume you want a clean sheet of it.)