I understand Black Salt absorbs negative energy and needs to be disposed of regularly to keep it’s energy strong. My question is, does it absorb the negativity before being put to use. For example, if I have it in my box of herbs do I need to use it the same day or can I use a week, two weeks later?
Terrific Tuesday Vicky,
I have not used black salt, so I do not know. But, thinking on it, I doubt you need to use it right away. Some things are stored for later. However, if you do not like the feel or your salt, I suggest consecrating it before you use it.
Lady Gene 
A lot of Witches Black Salt you can buy is not just salt it tends to a mixture of herbs, ash, resins and usually comes in a bottle or a sealed pouch. I have used in spell bottles which are sealed.
I am not sure whether putting it in with your box of other herbs would absorb any negative energies. I kept mine separate. Once you have used it either in a ritual or if you are sprinkling it around your thresholds once you feel that it has done its job then I would bury it in the ground a little distance from your home if you can. Alternatively, you can flush it away whichever you feel you want to do.
With love and light 
