Blanks do cost you points, but they buy you time. Or rather, blanks had better buy you time, or you're sacrificing points for no reason whatsoever.
Unfortunately, all too many students leave a question blank after they've spent a lot of time trying to solve it. If you skip a question immediately, you save yourself a minute that you can use to solve other questions, and you may need the extra time.
What you don't want to do is spend a minute trying to solve a question, and then leave it blank. Now you've lost 10 points and 60 seconds. An important skill you'll be developing is learning to tellat a glancewhether you should leave a question blank or not. If you can't immediately decide whether you can do a question, skip it for nowyou can return to it later.
Got all that? Leaving a question blank means not trying it at all. Once you spend time on a question, you've got to guess.