This is a beautiful combo - obviously H thinks so as it comes in So K, Evelyne, and Double Sens at least!
I disagree however that Bleu Saphir is a flat dark blue. It is simply a completely different color on different leathers. In Clemence it is nearly navy; in Chèvre it's like a bright jewel tone, like a real sapphire; in Swift it's richer/deeper but still a pretty saturated color; in Epsom like many colors it is quite bright.
Some H colors just vary more by leather than others
Colvert and Bleu Saphir are nothing alike IMO.