I found out that a call to blockingMapped can deadlock if too many pixmaps are processed at the same time.

It only seems to happen when the amount of processed images is higher than my threadcount and if the pixmap represents a real image, as it doesn't seem to work if I just create a red Pixmap of the same size.

So I added an example project with an example image.
I hope it's not just some facepalm bug on my end.