When I lived in Colorado, I never heard of blister beetles. When I moved to OK, one of the first things I heard was how 9 horses died at one farm earlier that year. I guess OK is the worst place in the world for them, particularly the Wa****a River Valley.
Ask your hay supplier. They will know if your hay comes from an area known for blister beetles or not.
Mowing with an old fashioned mower helps. It is the crimper on a swather that kills a lot of them in the hay. In order to keep leaves, almost all alfalfa is cut with a crimper. But, when a tractor tire runs over the fresh mowed hay, it can smash enough of them to kill a horse. So, there is no 'safe' alfalfa hay from an area that has them unless it is cut early in the year. Once the daytime temp hits 90 degrees, the beetles hatch out and are there until fall. Hot years are worse than cool, wet years. Around here, the only safe hay is first cutting. It is cut before they hatch out for the year.