I have debated this issue of 'holy' before. It can not be anything the gods create, or Diaboli would not be able to touch glyphs. So, it must be something created in such a manner to impart special powers on such an object. Perhaps 'holy' objects would have to have part of a god's very essence (or soul) imparted into them in a non-magical way, making holy or blessed objects not magical, but godly. As such, these objects could only be made under very special conditions, such as a god giving part of its power, or in a rite that steals part of that god's power. A third way could be to bring an object to a place that is already imbued with a god's power and absorb some of it.
Another way an object could be holy is if it is imparted with power from the people themselves in much the same way as the example above. A 'holy' man could place part of his own being in the object, dedicating it to a god. However, this -could- mean an object could be holy without being blessed or dedicated to any real god. Take Boon for example. He thinks there is a god of carrots, so learns the rights of how to make his hoe 'holy' by imparting part of his being into it. He does so, and now has a holy farming tool with the ability to harm the Diaboli.
Or, 'holy' might mean something all together different, and be a seventh way that people only assume comes from the gods.
Religion itself is -not- holy, or no Diabloi could belong to any religion.
Holy would then have to one of those. Power of the gods, power of the people, or a power appart from both, but assumed to come from the gods.
So... Is the Black Flame holy? If it is a god, then perhaps, depending on if a god's essence is what is harmful to Diaboli. If it is not an actual god, but a piece of Laanx's being, then perhaps it is holy, being a sentient object. If it is a creation of the people without anything to do with the gods (creation by belief), then it may be holy that way. In the end, it depends on what path the devs choose.