Black bitter substance when roasting chicken bones

I love making bone broth, and lately I've been taking chicken bones from carcasses that I've broken down and roasting them to impart flavor in the broth. The last few times I've done this I'm getting this black bitter substance coming out of the bones. It tastes bad and I don't want to add it to my bone broth.

Any idea what this is, or if I can prevent it form happening? I'm not cooking at really high temperatures (300-350F usually). I also don't get this when I cook the bones with meat on, only when I cook raw bones with the meat removed. I'm assuming the meat is protecting the bones from direct heat or something.

I should also add that the bones aren't burnt, I'm just cooking them until any attached meat is cooked through and some browning has occured.

submitted by /u/stevesteviestevens
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