Languagesystem Blac dflt;

The automatically generated “languagesystem” values are a combination of Script and Language tags. I’m getting one named “Blac dflt”, which is not in the OpenType spec. This seems to be an error, and I’m curious as to what character(s) presence has caused this to occur.

do you have some black letters (u1D504-1D537) in the font? Then Glyphs tries to generate a script tag for those. I have a look to prevent this.