Generally when you want your brows to be 2-3 shades lighter than the colour of your hair as it looks far more natural. You CAN lighten up brows to match your hair colour, but if it's lightened too much it can look silly. The only time I will lighten clients brows if they have gone blonde and have black brows, but you only need to lighten them to a nice medium brown, otherwise, it looks ridiculous!
My sister bleaches her hair blonde and she has light brown brows, recently she thought it would look better if she bleached her brows too...seriously, huge mistake! I had to end up tinting her brows for her back to her natural colour. Looks wayyy better.