http://www.sciencemag.org/news/2020/05/dark-lunar-eclipse-points-medieval-volcanic-eruptions

A total lunar eclipse typically appears red, but not after a large volcanic eruption.

On 5 May 1110 C.E., the night of a lunar eclipse, the Moon did not take on its normal reddish hue. Despite otherwise clear skies, it “was so completely extinguished withal, that neither light, nor orb, nor anything at all of it was seen.” So reported a version of the Anglo-Saxon Chronicle, a series of medieval manuscripts compiled in Old English. Now, by combining this report with an analysis of tree-ring records and ice cores, scientists have deduced the reason why the Moon became so dark: a set of volcanic eruptions in the early 12th century. The eruptions had widespread climatic effects, causing temperatures in the Northern Hemisphere to drop by about 1°C, the scientists report.