Mixing b/w and color developer should end in a very thin color negative, because the b/w developer takes away most of the color image.
Using a Blix should delete any b/w image from the film.
Using a b/w developer followed by a color developer plus using a Blix should end in a completly clear film. There is nothing left for the color developer.
The reason of getting a color negative could be:
- the b/w developer was dead
- time of b/w developing was too short or developer was too cold, so most of the image was left for the color developer.
The reason of getting a b/w negative could be
- Blix was dead or blix time was too short, so most of the silver image resides on the film.

I don't see any reason to use b/w developer, except of trying to do reverse processing.
ECN-2 film can be developed in E-6 color developer only, as long as the developer does not include the reversal bath. Developers from 3-bath kits cannot be used.

I developed lots of ECN-2 film in different developers.
Using homemixed ECN-2 developer gives the best natural looking colors. E-6 developer should create good results because it uses CD3 also.
Using C-41 chemicals creates some strange colors and much more grain.
Using RA-4 developer also gives good results, as long as the developer is fresh and was not already used for paper.
ECN-2 film gives low contrast. I mostly use it as internegative film for color slides.
I found out that 3 minutes of developing time mostly is too short. Negatives are quite thin. They can be scanned and digitally postprocessed but not printed optically.
So I normally develop between 5 and 6 minutes @100F which is much better for printing on RA-4.

Examples: Fuji Eterna 500T in 'Atomal' b/w developer (lake), Fuji Eterna 250T in homemade ECN-2developer (fireworks), Fuji Eterna 250D in C-41 (flowers)