For instance, the anime Darker than Black is R-17+ (violence & profanity) in the West, but is it higher or lower in Japan?
The television rating system in Japan is the same as the movie rating system.
Their regulations are as follows:
G: General audiences
PG12: Parental guidance suggested. Some content may be unsuitable for children under the age of 12.
These are regulated, and all movie theaters are required to check for ID. Allowing in an underage child is a criminal offense.
R15+: Restricted to teenagers 15 and over only.
R18+: Restricted to teenagers 18 and over only.
It is a lot more difficult to regulate television watching than to regulate movie-watching, so just because a rating is given for a television show, it doesn't mean that it is being followed, whereas in a movie theater, at least, it can be regulated. Frequently, in America, for TV-MA, a warning will be given that the show may not be appropriate for children under the age of 17.
Darker than Black was actually rated TV-MA in America, not R. R is only used for movies, and Darker than Black is a TV series. The rating is given here.
I cannot (at the moment) find what the Japanese rating is for Darker than Black, but presumably it was either R15+ or R18+.