I understand your question now.
Manajuma's knife does elemental damage, not physical. A physical damage weapon with the same specs would get more synergy with your boots.
Don't worry about that though. A physical damage weapon with a socket, high crit damage, high int, high lifesteal - is way out of your price range. Manajumas is probably what you can afford with all those attributes.
I recently spent $140M on a physical (black) damage sword that had slightly worse stats than my Manajumas knife, and it was a big letdown.
I have stuck with my manajuma's for similar reason to this. I have been looking for an affordable black dmg weapon for awhile now and there haven't been any out there. I have used d3up.com to check black dmg weapons that cost under 300 mill and none of them increase my dmg.
The reason mana's knife is so good is the garunteed stats that roll on it.(high critdmg, high int, high lifesteal) The only random stats that you need rolled are the dps and socket.
Rare weapons with black dmg have a much wider range of stats numbers and stat possibilities. You will see alot of black dmg weapons that are almost there but roll a low number such as 52 crit dmg or 2.20% lifesteal.http://us.battle.net/d3/en/profile/impking-1585/hero/3330596
You will still be getting the %elemental bonus using a "nonblack dmg" weapons like manajuma, it will just be a smaller amount. The base dmg of the knife, mojo dmg, and ring/ammy dmg benefit from the %elemental dmg from zuni trail.
It is very odd that the poison dmg on the weapon does not benefit from the %poison dmg on the boots. This seems backwards.
I would say manajuma's gory fetch(mojo) is not very important. If it has high avg dmg/crit chance then it will definately been a good offhand. Otherwise, you can use a different mojo.