Some stringmakers make their Fs black, and some blue. Most blue F strings are blue on purpose. I think I see see black Fs more on pedal harp strings and blue Fs on lever harp strings.
How young is she? I can’t imagine she’ll have that much trouble understanding that the “dark” strings (i.e. black/blue) are F’s. If she’s old enough to sit still for harp lessons, she should be able to understand that blue is the new black. 😉