Brick acid would probably take that off. Grout and pointing is alkaline so acid will work on it. White vinegar works too, probably a bit less gnar.
Wouldn't the acid react with the limestone first though?
Obvs depending on what the grout is, but unless it's lime based, I'd think it was less reactive.
@dbr started
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Brick acid would probably take that off. Grout and pointing is alkaline so acid will work on it. White vinegar works too, probably a bit less gnar.