What is so bad about box dye?
The chemical in the box dyes is very strong which can dry out your hair. I especially NEVER recommend box Dying your hair any dark tones or dying your hair any reds especially if you do plan on going lighter in the future.

The table on the left is the color we see on our natural hair color. Color on the right is the colors that are hidden underneath

I dyed my hair black and I want to go Blonde.
I get this a whole lot.. The reality is that it is not going to happen over night. From my experience it is really rare for the color to go blonde all in one sitting. Once you dye your hair black you may not see it but underneath your hair color there is a lot of stained reddish tones hidden from that Box dye you used 5 months ago. I can always tell when someone has box dyed or color'd their hair red in the past. How? Because once I apply lighter on the hair it will either lift to a stage 4 (red-Orange) or to a level 10 ( Pale yellow) . If you have box dye or dyed any red tones it will tend to stay at a red orange stage where as for virgin hair it will automatically go to a level 10 (Pale Yellow tone) easily.
I always recommend to cocktail lighten your hair and wait a months or two after to redo it another time. The longer you wait the healthier your hair gets and you definitely should use some sort of professional hair mask/ serum on your hair. My personal favorite hair mask is Macadamia Natural Oil Deep Repair Masque or Moroccanoil-intense hydrating mask.

How often should I wash my hair ?
The less you wash your hair the healthier your hair will get from its naturual oil. I used to be one of the girls that NEEDED to wash my hair every day but all honesty you have to trained yourself to wash it every other day .I notice a huge difference with the health of my hair Especially with the blonde I have in my hair.