This shirt looks to be one that the wardrobe department has dyed dark blue/violet, but that used to be a very light subtle plaid. You can still see the faint striping in the collar and on the pocket. I know the wardrobe department dyes plaid shirts dark blue, because as I type this I am currently wearing one such shirt that I bought at a SPN wardrobe sale at the studio. Much like Sam’s other shirts, this one also does up with snaps, in this case, the snaps are white (another clue that this was possibly once a much lighter coloured shirt.)
Sam wears this shirt for the entire episode of Bloodlines (9x20).