WordNet records no synonym and no antonym for venous blood. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
blood found in the veins
“except in the pulmonary vein venous blood is rich in carbon dioxide and poor in oxygen”
Synonyms, antonyms and the broader and narrower relations for venous blood come from Princeton University’s WordNet. Full sources and licences.