It was running up and down blades of Bouteloua gracilis at high speed even when I backed off.

There is only one image on the species page with the central black mark on the pronotum. All the others are red without a black mark. That one is from the Rio Grande Valley of south Texas, which is quite different from northern New Mexico.

Reference 4 on the genus page says that the legs of limbellus are pale except for the hind femora. I don't think that is true for any of the images on the species image page.