Blue stains from water
Blue or blue-green stains in fixtures is a sign of reduced copper. The source of this can be in the water itself (usually well water) or from the plumbing, usually caused by a very low pH (acidic water) which causes deterioration of the plumbing. 

This is an article describing the problem. You should get the water tested and have a qualified water treatment specialist or plumber come up with a plan to address it. 

Since you already have a water treatment system, it may just be out of balance.  In general, these systems require regular maintenance (flushing and changing of catalysts/chemicals).

Get it fixed. It's a health issue.