if there is a way to remove the blur when going into fullscreen
only one, and that is extremely limited:
if you make the original game size in a way that the fullscreen is a multiple of it, so that the resize is either x200%, x300%, x400% or so on.
If you don't have a full multipier, then the resize will have to guess colors for between pixels - that is what you see as blur.
Example:
you have an original with one black and one white pixel.
x200% would give four pixel, two black and two white. Everything Clear
x150% would give THREE pixel, one black to the left and one white to the right - but what color should the middle pixel have?
Most interpolation mathematix use a mix of the colors - black and white means grey for the middle pixel
and that is what you see as "blur", the guestimated colors on pixels that weren't there before.