Basil can cross pollinate. I am not sure the % of cross pollination you can have, I would guess that it might have to do with how many pollinators you have around at the time they are blooming.
I think that African Blue is a hybrid that doesn't produce seed.
I would collect the seeds after the flower stalks start to dry and turn brown, I would cut the stalks and put them on a paper plate or in a paper bag until they are dry. Basil blooms from the bottom up so it will mature that way too. I try to keep mine pinched so that it doesn't bloom until later in the season, but you can collect the seeds whenever you want after you have a mature flower stalk.
