This is a line from 'The Rime of the Ancient Mariner', a poem by Samuel Taylor Coleridge. The stanza containing the line reads thus:
'Her lips were red, her looks were free,
Her locks were yellow as gold:
Her skin was as white as leprosy,
The Nightmare Life-in-Death was she,
Who thicks man's blood with cold.'
The entire poem can be read here.