HOLD THE PHONES:
Okay, I have to admit something: I set the Blue 3 up using a Joe Grado decoupler like a do with all my Grados. Well, Marc was right: there were some cartridges according to his notes that it just does not work with. He used them on his older Grados and sang the decoupler’s praises. But on others, he noted flabby bass and a confused sound. The 3 series Prestige and the decoupled DO NOT MIX.
I tried the Blue 3 today sans decoupler and the high end was much more like the Opus 3. And while the voicing of the Blue 3 is much bassier than the Opus, it was far from the flabby mess I heard last night.
That out of the way, the Opus 3 still to my ears, trounces the Blue 3. The balance is much better for me. If you have a bass weak system, then maybe go for the Blue 3. While the high end cleaned up a lot, I still observe the Opus 3 to have a cleaner more engaging realistic high end.
On the other hand, the darker character reminds me a lot of the ZYX sound that so many people flock to. So keep that in mind. If the ZYX sound is your cup of tea, then choose the Blue 3 over the Opus 3.
Grain of salt: this is based on one listen with one of each cartridge. There could be wide variance sample to sample. Observations limited to my own particular cartridges on my equipment with my ears.