Blackflame Covenant + Varashta minions

This might just be me being stupid here but upon reading how the Varashta minions work, I was assuming that it would work with the "Blackflame Covenant" keystone. Turns out that does not seem to be the case. With the keystone applied, the tool tip shows only fire damage, the fire effects are also still normal and not purple.

To me this seems like a design flaw. I know I may be over simplifying this issue but it would be really nice to be able to convert the fire minions damage to chaos damage with Blackflame Covenant.

Thank you for a great league!