Alan,
I am sure the terms could be and probably were sometimes used interchangeably. But a farrior is a person who shoes and otherwise cares for the animal’s feet. 19th century Army farriors probably did actually make shoes at least occasionally (modern farriors mostly do not) but this was not their primary job. A Blacksmith would be more akin to a modern metal worker. Making horseshoes would be one of many jobs he would be expected to be proficient in. He also would be expected to be able to make any iron parts needed for artillery or wagons. Most (and probably most men in general) would have been able to shoe horses also. It makes sense to me for Cavalry and Artillery to have both these designations. Have you run across both in the same regiment/battery?