I want to give all the embers to the one near the church bell, but I've also met the other blacksmith in the catacombs just like he mentioned. Do they use them differently?

Each one can only use certain embers. So if they ask for it it's one that only they can use.

Thanks, just what I needed

@yoshimitz707 said:

Each one can only use certain embers. So if they ask for it it's one that only they can use.

What he said. If they can't use it, they'll tell you so and send you somewhere else.

Example: the burly white-haired blacksmith just before the Titanite Demon can make use of the Divine Ember (which is the only one I have so far).

I got an ember , and I haven't been back to a blacksmith yet.  Are they one time use items or do they upgrade the blacksmith's abilities?

@rubberluffy: They allow you to upgrade a weapon along a certain path. Giving the right blacksmith the Divine Ember allows you to upgrade your weapon along the Divine path. Use this link to see all upgrade paths.