As an online seller it helps your buyer substantially when you describe the color of the item they are purchasing because screen settings in addition to lighting settings (when photos of the item were taken) can make the color of the merchandise appear different than what it actually is.  And, since you are able to hold the item in your hands you are the only one who can truly describe the color, in words, to your buyer, so heres a great cheat sheet with 32 basic shades of blue to help you out.

There's hundreds (more like thousands) of different shades of blue, but here's the most common blue shades.  You can click on the image below to enlarge it: