To address a few points that have been brought up here:
dmg is calculated as a % of your MH in pvp
This is not true. It may have been true at some point, but I just logged in, queued into a BG to do a quick test. As of right now (9.2.0), off hand weapon damage does indeed affect damage on applicable abilities (or at least the tooltips). Test case was fury warrior, swapping ilvl26 cudgel vs ilvl 24 heirloom in the off hand with Blackout truncheon as a control in the main hand. That said, it isn't a huge difference we're taling about.
Just open askmrrobot and see the dmg formulas of dw spells
This is basically the best resource there is for checking to see if an ability scales with MH WDPS, OH WDPS, or both. You don't actually need to decipher the whole functional damage formula, you just need to see if it is using AbilityDamage(true), which means MH, or AbilityDamage(false), which means off hand. Some abilities use both, whether it be by actually having two hits (i.e. Stormstrike) or by using CompositeAbilityDamage (i.e. Pistol Shot). In either of these cases, basically 2/3 of the damage is calculated using the MH WDPS value, and 1/3 with the OH.
Abilities that scale off of TotalSpellPower for non-caster specs use main hand only.
Thank you for the reminder! From that spreadsheet, I got these formulas:
Mainhand or offhand abilities: 6*(weapon DPS)+AP+(3*Gems)
combined hand abilities: (MH Abiility+(OH Ability/2))*(2/3)
Are these the weapons' damage contributions to abilities?
The 3*gems addition threw me, especially since it doesn't appear in the combined ability formula. What is that?
The formulas are to put a value analogous to attack power on the total ability damage contribution from the weapons, so that they can be compared to other weapons in the same category. So that means adding the attack power contribution from the weapon(s) stats and the gems in the sockets to the WDPS contribution (where 1 WDPS = 6 AP). Then for combined abilities, I weight the average of the two numbers 2:1 in favor of the main hand.
The 3*gems is in there to account for having a +3 main stat gem in each gem socket. I chose this as a baseline assumption because it presents socketed items in the best light on the spreadsheet (which doesn't care about secondary stats or stamina). If you are using different gems, that does change things, but presumably you are using different gems because you think that they are better than +3 primary, which means you think they should perform better than (or at least as well as) the number on the spreadsheet.
You could do worse, but I wouldn't recommend the bladefist. The version that drops for 20s has no stats, and obviously no socket so all you have is the WDPS (which is the same as every other ilvl 25 1H) and the haste proc. By comparison
https://www.wowhead.com/item=27901/blackout-truncheon?bonus=6710&ilvl=25 has the same DPS, 3str/agi, a +1 AP socket bonus, 2 sockets, and a (smaller, but still substantial) haste proc with no ICD.
TL;DR:
https://www.wowhead.com/item=27901/blackout-truncheon?bonus=6710&ilvl=25 is basically the best available OH option. Next best would be
https://www.wowhead.com/item=27846/claw-of-the-watcher?bonus=6710&ilvl=25 if you cant use truncheon. For dagger rogues, get an ilvl 25 with a socket. WDPS does have an effect for off hand, but it's not all that significant. Gems too stronk.