Darth Aap finished up his plan but it needed a lot op preparation. He and the dark lords moved out to prepare. They were disguised (however you spell that xD) as refugees and with their force powers supressed the Jedi could not sence them. Before the plan was complete they had to vanish.
Darth Blaze could not join them. Not only was it safer to seperate, he sence something he hadn't senced for a long time. It was weak, but he had to check it out. If what he felt was right, it could change him forever.