I've never been able to figure that out.
However, my brain has overcome that cognitive dissonance by employing the following logic:
1. Red is red, and blue is blue.
2. Purple can be made by mixing red and blue.
3. A purple flower must have some blue in it.
4. Therefore, a purple flower may be said to be blue.
I dunno if you can call that logical.
Here is a Delphinium that I think is very blue-
This is Electra. Blue, or purple?