The box should probably read “Not possible ever with this formula unless you want red-orange hair”. Normally, drugstore permanent hair color is formulated to lift and deposit two levels of color or to add color matching picture on box. Your current color should be similar or close to picture when selecting “box” colors.
A color scale ranges from level 1 black to level 10/11 lightest blonde. So imagine how many levels of lift are needed to achieve colors in the blonde family.
The warning phrase “do not use on dark hair” has serious underlying meanings.
First, dark hair cannot achieve blonde results with a basic 20 volume peroxide ever.
Second, dark hair has underlying pigments of red-orange. Ever wonder why dark hair is left with an orangey red cast after hanging at the beach? Well, that’s natural underlying pigments. Orangey-red looks best at sunset not on compromised hair.
Third, the manufacture is warning in a subtle way “a mess up is highly possible if you think dark to blonde is possible in your bathroom”.
Lastly, if you are pumped about going blonde it’s a job for a highly trained professional to tackle. Highlighting and removing dark hair colors should be taken seriously as well as you committing to a home regimen to keep newly altered hair in its best condition going forward.