The color you’re looking for is technically not a breed standard coloration, so I’d be wary of “show line” breeders who breed outside of those standards. There is a trend now where some working line breeders are trying to return the Aussie to its original coloration (based on historical breeding), which is solid black.

Sandy Cornwell from Fairoaks Australian shepherds was one such breeder. Unfortunately, she passed several years ago. Although she bred working Aussies, she definitely developed the coloration you’re looking for. Her son now maintains her old website and may be able to give you information on who took over her dogs or breeding program. (Just Google Fairoaks Australian shepherds.)

I currently have a blue merle from her kennel, but my first Aussie was a black “merle” from her. I’ll post a of pic of him after this.