at the beginning of this quest, Martin says the Divines do not manifest themselves in Nirn, and have no Artifacts (Unlike Blood of the Daedra, where any Daedric Artifact will work), so we have to get Tiber Septims Armor to use the Blood from it.

But, what about The Brush of Truepaint? it is technically an Aedric Artifact of Dibella is it not? as its bristles are apparently made from Dibellas Hair

And how many people know that the Brush of Truepaint actually exists?

the CoC for one.

The Blades, and even Martin himself were hesitant to acquire Talos's armor from Sancre Tor. the CoC could have made it easier on everyone saying "Actually, theres a Paintbrush in Cheydinhal that will work just as well, I`ll be back within the day"

Not if you haven't done that particular quest yet. Or at all. It is optional, and some players might not find it.

but at the same time, Blood of a Daedra allows you to use any Daedric Artifact, even though the quest points you toward Azura's Star.

So, wouldn't it make sense for us to be able to use a separate Aedric Artifact if we know of its location?

Difference might be that the daedric artifacts stay with you once you earn them.

I don't remember ever getting the Brush. Getting it to give back to Rythe, yes. Keeping it in my inventory, no.

The Black Horse Courier has an article that make it sound like he's still painting.