Can I blow my nose even if a few minutes ago the mucus had blood in it?

You are probably blowing too hard and breaking small blood vessels in your nose. You should try to be more gentle, blowing your nose-- try using steam or a hot compress before you blow your nose to loosen up the mucus, making it easier to blow out.  Also, try blowing only one nostril at a time.  You do not want to blow your nose so hard that it causes a nose bleed.  If your nose does start to bleed, pinch your nose with your thumb and index fingers to get it to stop bleeding.  Use a hot compress or steam before you try and blow it again. Also, make sure you drink at least 8-10 eight ounce glasses of water when you are stuffy. You can also try a NetiPot or nasal wash product.