all depends on what kinds of case you are talking about. for TPU cases ... i think you will see the stain in the first few days... for PC or hard cases with coatings.. shouldn't have any stain..
You need to wash the Jeans until the excess colour runs of them. If they are new that will be the cause, if not, then I guess another case would be a good!
You can also wash the bumper to get the Blue colour off!