If the block is traditional and you know the name, then use that. If you don't know the name or if it is an art or modern quilt, then just say 'free form', 'unknown' or 'none'. The description fields help others when searching to find quilts, i.e. 'pinwheel' search will bring up quilts using that block. There is a field further down the list for genre that includes art quilt as a choice.
Kathy