Synonyms for Blackthorn (same or very similar meaning)
WordNet sense 1 (a thorny Eurasian bush with plumlike fruits):
sloe, Prunus spinosaWordNet sense 2 (erect and almost thornless American hawthorn with somewhat pear-shaped berries):
Crataegus calpodendron, Crataegus tomentosa, pear haw, pear hawthornFrom the ODE community, based on WordNetadd/edit