The actual screenshot taken should be an exact copy, but when SC renders the image in its window and has to SCALE it down, it can appear blurry.
So first thing to do is confirm this, by switching to 100% view inside SC window (which will make the capture larger than the window but should confirm that it looks fine).
Then you can go change the scaling algorithm in SC:

Let me know which smoothing option looks least blurry.. Maybe I can make that the default.