This sinister-looking dagger is a drop off of Warchief Rend Blackhand. Its model is not unique, but the coloring is and I highly recommend farming for it despite its low droprate. (UBRS is being "revamped" for Warlords of Draenor, so there's a possibility this weapon will become unavailable.)
The drop rate of this weapon is somewhere between 1-3%.
As of writing (patch 5.1) this dagger is largely better than many level 90 weapons in the OH of a rogue. The proc has no ICD and an estimated 5% proc chance. Combined with a powerful MH weapon the proc does some serious damage, especially as assassination as poison ticks crit. With some combat dummy tests, I've discovered that being hit capped is required for a DPS gain, also the weapon scales better with gear than most daggers. Making this potentially a good choice come 5.2 once new raid gear is obtained but new daggers are not yet obtained.
I am not sure what you mean by your comment. Either you meant item level 90, in which case MoP content is irrelevant to the matter, or you mistook this dagger for a MoP weapon.
Edit: OK, you were talking about the proc. I can understand where you're coming from, but it would not surprise me in the slightest if its proc rate was changed to be reduced at levels higher than 60.