Keep their cages close to each other, but not so close that they can bite through the bars; dont let them out at the same time together, yet. Keep them like that until Bell is spayed (its still considered bonding since they see and smell one another). I would also recommend getting a small stuffed toy or small towel to throw in each of their cages. After a few days, swap the toys/towels into the other bunnys cage. Itll get them used to being even closer to one another
When it comes to mounting, you want to pull the top bun off after 15 to 30 seconds, or when the bottom bun starts to look irritated. Separate them if you sense a fight. Only let them meet face to face in neutral territory. Its likely Blue (i have a Blue too!) is mounting her because he can smell her hormones, and because he was there first so he needs to prove to her that hes the boss. She is likely mounting him back because shes mad he keeps bothering her, and because her hormones are starting as well. Ive read that female rabbits are often dominant over males, so she may be mounting him to show he is no longer the boss now that shes here