"A Djinn blade is a power weapon." This is a quote from its entry in the Weapons section of the Wargear pages. It means that the Djinn blade is only different from another power weapon in its fluff, and the extra rules given after "Furthermore". Therefore, a Djinn Blade is a special close combat weapon, as Buffo has stated.
Torcano, the fact that it doesn't replace one of your original two weapons means that you can replace one of them with a blaster without losing an attack, which is a third reason it could have been listed seperately from the other weapons. It still begs the question, "Why only the Djinn blade?" though.
Unfortunately, the BRB page 42 has this to say on wielding two different special close combat weapons, "When it is their turn to attack, these models must choose which weapon to use that turn, but they NEVER get the bonus attack for using two weapons". I think this means that even if you have a Venom Blade, Splinter Pistol and Djinn Blade, you have to choose which special weapon to use, and don't get an extra attack for having two close combat weapons regardless of which one you choose.
As whether you get your two bonus attacks if you're not actually attacking with your Djinn blade that round ... I admit that I'm unsure on this. Unlike most weapons with a special effect, it doesn't state that the effect applies only when attacking with that weapon. For example, the Huskblade rules state that it "is a power weapon that causes instant death ...", and the ability of the Mindphase Gauntlet only effects a model "that has been hit by a mindphase gauntlet". The Djinn Blade's rules only says that "the bearer makes two bonus attacks every round of combat". It doesn't state that the bonus attacks are only made if the "bearer" is making his normal attacks with the Djinn Blade.
If you assume that you do get your bonus attacks when wielding a Huskblade for your normal attacks, are the two bonus attacks Huskblade attacks? It states that the bearer makes two bonus attacks each round. It doesn't state that they must be made with the Djinn Blade. I don't think that this was the intention of the designer, but I always play by RAW to avoid confusion. I can't decide what RAW is in this case, so any further input would be appreciated.
I wish they had just made it a "replace one of" weapon like the others, and made the blaster a "may take a" option.
<edit> ... all this confusion would not exist if the statement "A Djinn Blade is a power weapon" was not part of the entry. Is this, perhaps, an error? It makes sense if you assume that the Djinn Blade entry was copied from The Lady's Blade, rather than the other way around. If so, hopefully it will be corrected in the FAQ. In the mean time I'll just stick to my delicious Haemies and the Duke ...