I think maybe you should consider draining your oil and changing your filter asap too.
If the black grit did come from your cylinder head after machining, it is likely to also be in the oilways.
The filter would have caught it, but if we're talking any quantity here, it could begin to fill up your oil filter.
I wouldn't mind betting that if you cut open your oil filter, you would find similar ferrous grit inside.
The machine shop should have cleaned your head before refitting it, and this grit must surely be a normal by-product of the machining process.
Doesn't sound good practice by the shop.
1994 E200 Wagon 115k miles