Sentence:  Unlike some of his contemporaries Parkes never implied that the Irish were close, in the racial hierarchy, to black, condemned to some racial <WSD>penumbra</WSD>, between black and white; but nor, given Catholic exclusion from the given traditions of his native radicalism, were the Irish white in the same way that he was. 
Meaning of the word marked with <WSD> tags:  An area of uncertainty or intermediacy between two mutually exclusive states or categories.