Eureka with the blue

[Kisshu]

in all likelihood the devs don't want BLUs "ruining" people's previous BA clears (which could be mitigated by just having it give BLU achievements when playing a BLU rather than the normal ones but whatever), which iirc was the cited reason why BLU doesn't get to even attempt ultimates content.

In such a case it would really help if they communicated a bit more with the players compared to what they're doing now. If they tell us they want to limit BLU's access to Eureka because of BA, then players could perhaps persuade the devs to allow BLU in the zones, but not BA. But at the moment we have no idea why BLUs are excluded at all.

At some point they're going to have to repurpose Eureka like they did with Diadem.
It would be perfect for BLU because the enemies have elemental resistances pre-programmed, and can be played solo or in a party.

To be honest, I don't think it's programmed resistance/weakness based on the type of damage the player does. BLM and RDM deal fire/lightning/ice/wind/earth type damage and doesn't have any effect on damage that I recall.