How do I color bleached blonde hair back darker

You need to pre pigment the hair to avoid unwanted khaki colour. It's best to get colour correction done professionally.

Keep in mind that dark hair contains red pigments in it. And when you bleached your hair, it stripped away all the pigments, including the red tones. Therefore, you will need to put the red back in your hair to prevent your hair from turning green or an unpleasant shade. It's much easier to go darker than it is to go lighter, but you have to use the right base color. To go from blonde to dark, you will need to use a reddish brown hair color first and then apply the desired color.

Yep, I agree! When doing anythign correcting OR drastic, always seek proffessional help as if you mess that up too, then you can't keep on correcting it. It's always best to pay that little bit extra and to have peace of midn that you haven't caused that much damage to your hair!

Just be sure that you do want to go back dark for quite a while as you can't really keep bleaching if it's not what you want.