[B]listers [first] appear as reddish or pinkish swellings, later as rough red swellings on the lower surface of the leaf blade. Often they coalesce, covering a large area of the leaf blade alongside the midrib.
As blisters become infested with mites, they turn dark brown to black, and by the end of the growing season the affected tissue has become dead and shrunken. Blisters ... not colonized by mites are pale green ... [B]listers without exit holes never contain mites.