Thank you all very much for the replies and suggestions! The skillet in question is a Griswold #10. I originally used Lye-based oven cleaner, and 50/50 vinegar and water solution to clean and de-rust, then used Pam to season in 500 degree oven (twice, per Cheryl's instructions posted to this forum). However, I did not leave the skillet in the vinegar water solution very long, as it did not really have any rust. I probably only left it in for about 30 minutes.
Russell, I did wash off the oil from the skillet, paper-towel dried it and left it out, and it stayed black. So it sounds like I can either go ahead and use it, and let it build up some additional seasoning to hopefully reduce the shedding, or I could re-clean and leave it in the 50/50 solution a little longer this time and see if that helps?
I am relieved to hear that you all think the likely culprit is carbon residue, which I'm assuming is not too bad of a thing to consume. :)
Thanks again!
Paula