It worked! I went back to the dyeing table, and I repeated the same ratio (1 part Boysenberry to 3 parts Turquoise) but this time with very hot water. The turquoise stuck around through the "fixing" process and the result is a really good match.
It's so perplexing, Helena. Your instructor, Ann Johnston, told you to stay away from Turquoise. When I took Elizabeth Barton's course, Dyeing to Design, she specified Turquoise as one of the six primary colours we were to use to create all other colours. It was to be used with the two other cool primaries, Fuchsia and Lemon Yellow.
Time to move on, I guess?