Brooklyn is correct. If you henna by itself you will have a reddish (even orange sometimes) tint. If you indigo by itself I think you run the risk of blue or green hair. Henna and indigo WHEN USED TOGETHER at the same time give you shades of brown I believe (I do not have my notes handy). Indigo OVER henna, applied after you've done a henna treatment will give you the black you're looking for.
You can use a gloss at anytime, it's just henna mixed with a conditioner (and whatever else you want to add). You get less intense color with some of the conditioning benefits of henna and the other ingredient(s). I suppose you could use Cassia to do a gloss but in that it's colorless, I'd use it by itself for maximum benefit.
HTH.
Try PMing Sareca, she's much more knowledgeable about the indigo than I am. I'm a hennaologist, she's and
hennindigologist.
She can come correct any misinformation I may have provided (my journal got wet yesterday so it's useless right now.
)