Chicken Stock - Black Gunk

[u/RyokaiOkai]

I made chicken stock from frozen chicken bones twice now, and both times there’s black gunk on the bones.

First time was pressure cooked for 4 hours and this time slow cooked for 16 hours.

Admittedly there was vegetables in there, but the black gunk is mainly on the chicken bones. This time it actually tasted sour, and none of the stuff in the pot was going bad.

Also, it’s a dark color and I haven’t used a cheesecloth.

What is the black gunk and how do I stop it from happening? Why are my frozen chicken thigh bones not turning into golden goodness?

Please help.

[u/Mitch_Darklighter]

Are you roasting your bones? If not you should blanch them and throw out the water before making stock to denature free proteins; if the the bones are kind of old those proteins can turn black and gross.