line of descent vs blood line

line of descent and blood line both are nouns.

Word	Noun	Adjective	Verb	Adverb
line of descent	Yes	No	No	No
blood line	Yes	No	No	No

As nouns, line of descent and blood line are synonyms defined as:

line of descent and blood line: the hereditary derivation of an individual

Other synonyms of line of descent include ancestry, blood, bloodline, descent, line, lineage, origin, parentage, pedigree, stemma, stock.

line of descent (noun)	blood line (noun)
the hereditary derivation of an individual	the hereditary derivation of an individual
the kinship relation between an individual and the individual's progenitors