My guess is that you're using a feathering tool, airbrush or some other kind of pen/brush option with a blurry edge. That's just a guess, but if you're drawing straight to the computer, that's the only thing I know that will cause blurring at 100%. If you're drawing by hand and scanning, my guess would be that you're not scanning at the right size and you've been changing the image size with the cursor. Again, just a guess based on no information whatsoever. Tell us more about your process so we have a better idea what to suggest.