Black apple crumble!
Walnuts are most likely your culprit.  Black walnuts have more of this tendency than common cooking walnuts, but I have had it occur with both.  I especially have seen in with port chops stuffed with walnuts and applesauce getting a very purple stain to them.  I have seen multiple theories on why purple, but with walnut breads a common suggestion seems to be that iron reacts with the walnut to cause the purple, and some flours are higher in iron than others.

And idea, but just a suggestion, is toasting the walnuts first might reduce this tendency.  In fact, here is an article from Cook's Illustrated https://www.cooksillustrated.com/how_tos/6672-preventing-purple-walnut-bread.  They indicate that they believe it to be Gallic acid in the walnuts doing it, and that blanching for 1 minute before using them will reduce the effect.

ETA:  Looking up Gallic Acid was interesting.  It was used as a key ingredient in "Iron Gall Ink" in the 12th through 19th centuries as a standard writing and drawing ink, giving some idea of its ability to color.