I don't know the reason for this. But I would proceed as follows:
1. print a black and white document and make sure that no colours are used. The best way to do this is to print a pdf document explicitly in greyscale.
2. remove the blue toner and see if the problem still occurs. However, I don't know if printing is possible if there is no toner in the device.
Of course, it could also be due to the image drum.
And here is some hopefully helpful information:
https://www.justiot.de/infrastruk...