It could be that the underside of your head has a clump of ink on it and this is dragging across the paper. Contact http://www.TheServiceManualShop.com for there document about head cleaning - the document is not listed on the site but they can email it you. If this is not the issue then it could be that a cart has leaked into the cart carriage and is colouring one of the other inks. This is not that likely however as blue is a primary colour and can't be made by mixing inks. If it was for instance green bands that it would be plausible that blue ink was leaking into the yellow or vice versa. Another option is that the piezo thing that actuates the ink being fired is capput and allowing blue to leak through to the nozzles all the time. Or that the print head or main board is faulty. I'd try the cleaning first - it's the easiest option.
Usually answered in minutes!