If you increase the size of a sprite you need to do so by increments of 100% (e.g. 200%, 300%, 400%). If you increase the size of a sprite by a different factor (such as by 150%) the program has to interpolate sprites causing the blurring.
you just need to turn off interpolation. Go to global game settings, then the platform your testing on (probably windows), then go to graphics and click the box for "interpolate colors between pixels" to be off.
So your resizing them inside the editor? And its creating the blur? Try using something else such as Paint - there's a resize setting that works... Dunno if GameMaker has it
GameMaker does have it... Press Ctrl+Alt+E and select "Excellent" for quality