I accidentally sent some c-41 film into the lab one time, thinking it was b/w. oops
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came back as b/w color negs. scanned ok. contrast was really high though, highlights/shadows totally blocked up. I wouldn't recommend it though. If you want b/w from color film, the Ilford XP2 Super/ Kodak BW400CN are nice films, and print nicely. if you print b/w only(on b/w papers), the XP2 would be the better choice, since there isn't the orange mask. With the BW400CN, there's the traditional orange mask. the kodak one is flatter in most cases from what I've found.
for b/w only though, I'd personally rather shoot real b/w. unless you don't dev your own film/take it to a lab.
-Dan