Someone told me to lay a quilt out on an old sheet, fold it over and put in a bathtub with mild detergent and soak briefly. Then let the water out and add rinse water, several times. Then roll up to gently squeeze the water out. Spread flat to dry. Hope this helps.
It has also occurred to me that you don't know if the fabric has been washed...and the blue could run. I would check an edge before doing anything. It doesn't appear to have stains, so going ahead and quilting it without washing is also an option.