The djin blade is, by RAW, like a servo except that:
A) You can use it as a regular power weapon *too*, if you want, though you could also use a different weapon; and
B) It apparently uses the bearer's S and any special rules they might have, such as re-rolling wounds from combat drugs.
However, note, you can't combine the effects of different weapons, and you must choose which to use each round. That's a core rules thing. So the only part of the djin blade rules that confuse me are the question of whether, to use it, one must make all attacks with it, including the two bonus attacks, or whether instead you can make primary attacks with another weapon and then two bonus attacks with the djin blade in addition.
Of course, this won't ever come up for me, because I'd never buy, like, a djin blade *and* an agoniser. It's way too expensive and they have no synergy, no matter how it works. If I went with djin blade I'd just, well, have a fuckton of power weapon attacks and hope I rolled either +1S or re-roll wounds for drugs.
(Note with a djin blade, as it seems you can use a pistol with it, an archon can make up to 8 power weapon attacks on the charge, or 9 if he got +1 attack from drugs. That's a lot. It's not a bad weapon, really, and not *too* risky if you have a shadowfield to go with it.)