I keep getting a blotchy purple interference on my photos in the Develop tool in Lightroom. It seems to be pretty much entirely independent of the photo (i.e. from a cell phone vs. from a DSLR). It is worst when I edit shadows and blacks in a photo, and sometimes I am unable to edit at all without causing the photo to turn purple. I have tried resetting the settings to see if that helps, and it does not seem to solve anything. Has anyone experienced his before? Any suggestions?
You have the clipping indicators active, and the blue you are seeing is indicating that your dark areas are clipped. You can turn the clipping indicators on/off by pressing the J key. Otherwise, you can adjust the image to eliminate the clipped areas. Clipped means that the shadows are too dark and are losing detail. The highlights when clipped will show in red.
It is possible to turn the clipping indicators on/off individually. There are little squares in the upper corners of the histogram. You can click on those little squares to change the activation status of the indicators. This square on the left controls the shadow clipping indicator, this square on the right controls the highlight clipping indicator. Generally speaking, I leave my clipping indicators active as a guideline in my postprocessing to ensure that I adjust to what I consider to be proper settings.