String instruments have black lines around them but they are not just decorative. It is called purfling and it is actually strips of wood put into the instrument to help the structure. That is one characteristic of a quality instrument. Painted on lines can be a tell-tale sign of a less than desirable instrument. Although the function on a plywood / laminate bass may be debated, the purfling is often skipped or faked on cheap instrument. Not all basses have purfling, and this isn't a make-or-break item, but this is another attribute to notice. Here are some pictures of purfling on a violin.