Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ŏbumbrātĭo, ōnis, f. [obumbro], a shading, darkening; trop., an obscuring (postclass.): allegoricae caecitatis obumbratio, Arn 5, 186: vicissitudinis, Vulg. Jacob. 1, 17.