Black gold does exist. Boucheron used to makes some spectacular
pieces using it. I asked my teacher about it when I was still in
school and he told me it was made from a gold/iron mix and it was
Like "blue" gold, black gold is not an intrinsically black alloy.
It's greyish white. But it can be given a nice black patina. Argen
metals carries an 18K alloys they sell as "black gold". Coloring it is
a heat treatment. Hot enough, by the way, that stones other than
diamond probably would not survive. Their "black gold" is a nice gold
color, so is probably not based on just iron.
The gold iron alloy is not what I'd call "fragile". it's just quite
brittle. Rolling it or drawing it has to be done gingerly. But can
still be done. For "fragile", try purple gold, an aluminum/gold alloy.
That's so brittle and glass-like that dropping a piece can shatter
it. Unlike black or blue colors, is an intrinsically purple alloy,
not a surface patina.