Cover Your Roots
As they grow, are the roots of your bleached blonde hair beginning to reveal their black, natural color? Dab on hydrogen peroxide and leave it to rest for half an hour or so, then wash it out. This frequency is enough to mask her roots (think of Margaret Houlihan from MAS*H *, who still wanted to cover her roots up by using the hydrogen peroxide she had stashed from medical supplies).
Hydrogen peroxide can be found in all sorts of coloring/ dyes used to color our hair. It has been used widely across the globe and has given proven results also.