I have just tried a green diraquine scimitar and it adds light damage indeed.
Not acording to my calculations. Wither makes blind and hex apply twice: damage x 0.5 x 0.5 = damage x 0.25 (75% reduction). Therefore, with a 66% blind effect the final effect would be: damage x 0.34 x 0.34 = damage x 0.1156 (88.44% reduction).
However, the issue seems to be that the Diraquine 6 set bonus isn’t working at all. I have tried with an epic 6 piece bonus and blind only reduces weapon damage by half intead of the 60% stated.