Well if your goal is just to play fingerstyle blues, and you're not really into the classical, don't grow your nails. The techinque will apply, studying classical will get your right hand coordinated, enable your thumb to play bass lines while your other fingers play melodies, and overall make you a competent fingerstyle player in any style. It will make your left hand stronger for things like slurs, etc., though the positioning is a bit different because blues requires you to bend. I say take the classical lessons, you'll learn something either way.