I would consider C99 and Blueberry stable but I’m no breeder.
You seem interested in Bros. Grimm. From my experience you can find a keeper from a pack of C99. Personally if it was me I’d go for the cheaper Apollo 11 or Apollo 11 haze. Again you find a keeper with little effort.
If you really want C99 get it from FDM and grab a pack of his new Sweet tooth #4 for less than you’ll pay for Bros. Grimm C99. The Sweet tooth provides 2 desirable phenos to choose from.
Fleur du Mal Seeds - #163 by Muleskinner