could (do something) with (one's) eyes shut

Could do something with very little or no difficulty, or complete or accomplish something in a relaxed, carefree, or effortless manner. It took me a little while to get used to this job, but now I could do it with my eyes shut! You should ask Johnny for help—he could do this level of math with his eyes shut. I'm a professional musician, so that song is hardly a challenge—I could play it with my eyes shut!