I think last 5th anni blue robe i saw that was marked and unblessed sold for like 650 or 700k which seemed pretty low. But also some of the 4th anni red robes recently went for around 500 or 600k. I think given that it's unmarked but blessed i could see it fetching 500k or so. I'd bite on it right away if the SB was 400k.
It’s a hard item to price. You can’t cut it so that market is out. It isn’t anniversary marked so the ‘collector’ may not want it for their sets. It looks great but it would be for a small market.
I paid 900k for my marked/blessed 5th robe a few months back and feel like i got a good deal. Unmarked, you need a special audience that more interested in the hue than the markings. Personally i would value it around 500k. But that's just my $.02. Its a great looking robe and it is one of the best hue's.