I've been using Char Y and I can attest to this. However, I would think that Hidden Power Ground would be better instead of Focus Blast, if your issue are Flash Fire users. Hidden Power Ground hits Heatran for the same as Focus Blast, but with infinitely better accuracy. We have coverage for Ice and Rock 'mons already, and as for Dark mons, I can't think of any that would withstand any of ZardY's other moves except for Ttar and maybe Mandibuzz, but that thing isn't even fazed by Focus Blast. As for Ttar, I mostly just call it a day and switch to my other mons that can take care of him.
On a different note, I've been using a Rayquaza-mence offence, but using ZardY-Kyurem-B instead. They resist each other's weaknesses bar Rock, and the damage they put out to the table is monstrous.