Occasionally, we'll get a message from a customer about their print being "stained" or "mis-printed."
The background of the image is an authentic vintage blueprint page -- so it has mottled ink colors - patches of darker and lighter, just like old blueprints used to:
(The really sharp-eyed will notice a very faint gridline in the background, as well.)
The page is intentionally mottled with darker and lighter ink patches -- exactly like period blueprints were. In fact, from a graphic design standpoint, a uniform, flat blue across the whole background would look very 2-dimensional and boring; the changes in ink saturation give the image some depth and make them more appealing to the human eye.