TL;DR: Iris West was always caucasian in the comics and the fact they're portrayed by African-American actors is simply a diversity gesture from CW.
The official "casting call" description provided by the show's writers goes as follows:
IRIS – 22-28 – African-American – As smart as she is beautiful, Iris
is in grad school studying psychology. She’s also Barry’s mile a
minute, fast-talking, quick-witted best friend. Her father, Detective
West, took in Barry when his mother was murdered, and his father was
wrongly accused and imprisoned for her murder. In a tough childhood
for Barry, she was the one “not tough” thing. She’s unaware of Barry’s
strong feelings for her.
DETECTIVE WEST – Late 40s to Early 50s – African American – Detective
West is an honest, blue-collar cop who’s seen it all. A soulful, funny
caring father to Iris, and a surrogate father to Barry, West came up
through the foster system himself. He took in Barry after his mother’s
murder and his father’s imprisonment. He believes in Barry and
supports Barry’s efforts to prove his father’s innocence.
The reasons behind this choice are beyond me, but it did add a bit of details to the life of Barry Allen after the mother's accident, which was never fully addressed in the comics, and by introducing detective West and the adoptive family all together, it seems more undertandable how Flash didn't turn into a full Batman or Green Arrow since he was surrounded by a family.
PS: Even the flash isn't blonde in the series.