This is not the first example of "reverse graffiti" seen on the Tube.
I spotted the above at Aldgate earlier this year, which obviously isn't as "on brand" as the roundel.
Wonder what other messages will appear in the dirt? Although maybe this is a sign of things to come. Perhaps TfL could make some revenue and save on cleaning costs by sponsoring "dirt advertising spots". Just wish I could think of a better name for selling it to them. Ideas most welcome!