Much detailed information is only available in English:
The Black Breasted mutation changes the pattern of the markings.
This is an autosomal recessive mutation.
This is a Black Breasted pair.
Black Breasted male.
This is a Black Breasted Black Face Fawn male. Ideally, the breast and belly should be much darker.
Because Black Breasted changes the pattern of the markings, rather than their colour, it is very interesting to use in combination with other mutations. Try these for instance: