Dark areas on bottom of yeast starter.

This is a weird one for me. I made a yeast starter a couple of days ago for a beer that I was supposed to brew this weekend, but ended up not brewing. So, I put the starter in the refrigerator to save it for another brew.

I noticed these black areas on the bottom of the flask as it's settling out. Anyone know what this is? Is it trub?

Everything in the process of making the starter went fine. Added a drop of Fermcap-s and some White Labs yeast nutrient, pre-boil, like I always do.

Thanks in advance!

How did you sanitize your flask?

Looks like you maybe scorched your DME when you were bringing it up to a boil?

Just for the record, it was scorched. Don't make a yeast starter late at night when you've had a bit too much to drink...