My husband had to vigorously plunge a toilet in our upstairs hall bathroom the other day. That afternoon, I noticed the hot water handle in our master bathroom sink (which has a shared wall with the difficult toilet) had black liquid oozing out of the seams. We've cleaned the sink and it keeps reappearing every time I turn on the hot water. The black liquid trickles out from the handle. My husband thinks it's related to the plunging, as the toilet is likely right on the other side of the wall. I'm not sure what it is and why it's happening and only to the hot water handle. I have images, but it doesn't seem this forum provides the opportunity to share images. I'm grateful for any guidance you can provide. I googled what i could and haven't turned up anything that sounds similar to what I'm seeing. It's not clumps or flakes and it's not coming from the faucet. It's a thick, evenly colored black water (as if I rinsed a paint brush with black paint) and only coming from the handle that controls the hot water. Thank you!

The two are in no way related. A photo would be helpful. It is probably residual from the stem packing along with water mixing with it.

yeah, it is the packing deteriorating. usually a tightening of the packing nut will fix it. old style packing had graphite in it..
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A classic case of two events occurring simultaneously and someone assuming they are connected, which they are NOT. Tighten the bonnet of the faucet to eliminate the small leak that is causing the black water to come out.

Where do you live in California...not Taft, I hope. Take a photo to your water provider if it persists after you tighten the bonnets of the valves.