what is the point of dervish blade?

It's because it's only allowed in Twisted treeline and crystal scar. Those maps have a few different items then SR and ARAM.

It's pretty nice late game on some champs, perhaps like Irelia, or ranged ADCs, particularly ones that are more auto reliant.

Technically, it's a bad Mercurial Scimitar built for maps where champions don't have access to Mercurial Scimitar.

Also, keep in mind that on Crystal Scar and Twisted Treeline, the game is a lot faster and it gets harder to build super expensive tier 1 items like BF Sword, so on that map Dervish Blade might be a really good option compared to Mercurial Scimitar. In addition, ADC's as a whole aren't as good as other kinds of AD's, so Dervish Blade might give them a boost that other kinds of AD champions can't really get.

it's only for TT and domninon, and BF sword doesn't exist in those maps, so the BF items are replaced by different items, that are often cheaper or have an easier builtpath. Why they decided that it should give AS rather than AD is beyond me though.... A sweet item if you face malzahar as irelia though.