It only seems to have just started happening but whenever I take a screenshot (shift, command and 3) when I open it, it is blurry. i have to zoom right into the screenshot and then zoom out again for it to be clear.
This is a known issue with Preview in 10.11 that is still present in 10.11.2 - it is failing to render images correctly, leading to blurriness and poor quality. For now, you have to zoom in or drag the window size slightly to force it to re-render. You can also try turning off "Smooth text and line art" in Preview preferences.