The damage of Blackfires passive is "meh", as flat scaling is still stronger for the most part. People are over re-acting to casters finally having decent items. The reason everyone is spamming them is because all other caster items are terrible in comparison, they do not give AP's what they need to do damage/survive in dominion.

Blackfire's damage dot is the same as the base dot on a level 3 children of the grave, a dot so weak in damage that everyone outheals it with their base health regeneration. The passive is highly overrated.

Ghostblade is internally synergistic, of course, but it's considerably weaker than BFT, adding an item with counter-synergy is not amazing though.

And comparing two items to two items is a whole different kettle of fish, because the number of combinations is very large and thus the conclusions are often inconclusive.

As for the scaling, triforce doesn't really scale, and it's strong. BFT has its own arpen with it, making it superior to most items which deal magic damage, that don't include mrpen.
For the cost, the item gives you everything you can ask for, nearly, and in good amounts for the price asked. I always find ghostblade underwhelming, honestly and get it as part of a "full build" since I already get brutalizer early.

even with 0 mpen it ends up doing more damage than bft(does not only tick once per second).

the passive is overrated, but with all the stats combined it's a little on the cheap side.

Ghostblade isn't considerably weaker than BFT, its about the same. However under the current S2 system, the value of flat magic pen is greater than the value of flat armour pen after midgame, so ghostblade isn't as good endgame as BFT is.

When the S3 changes kick in, expect to see a sharp uprising in Ghostblade users (and that new Black Cleaver) for the above reasons.

Yeah.. you know i've been using the same argument about Pillager since it was released but everyone wa slike naah pillager is fine... it lets me kill evuuurything while mages sit at a needlessy useless rod, but then bft come with 2 stats less than pillager and everyone is complaining?, screw logic i'm going (adult movies business)