Q: Do certain races have better ability at channeling than others? For example, the Sea Folk at very good at weather, the Aiel have a high proportion of Dreamers, and the Seanchan can make ter'angreal.
RJ: The Seanchan can only make one kind of ter'angreal. They haven't thought about making another. Certain groups are better at some abilities but its a matter of need. The Atha'an Miere are dependent on the sea, the wind and water and it would be natural for them to develop high skills to deal with control of weather and winds. For the Aiel, Dreaming is one of the ways to find new water, using need is how they find water. When the population in a hold is too great, and they have to find a new hold, the Dreamwalker uses need to find it. So yes, there are more Dreamwalkers there.

The sentences I emphasized speak against the idea that the Seanchan created the Bloodknives' stone ring ter'angreal. On the other hand, the text in TGS seems to support the notion that the Seanchan created those rings: "The pure black stone ring each one wore was a specialized ter'angreal that would grant them strength and speed, and would shroud them in darkness, allowing them to blend into shadows."

Either the Seanchan found those ter'angreal (though why wouldn't they copy them then to have more?), or BS simply changed RJ's original intention that the Seanchan only know how to create the a'dam ter'angreal.