The Collaborative International Dictionary of English v.0.48:

Blackthorn \Black"thorn`\, n. (Bot.)
      (a) A spreading thorny shrub or small tree (Prunus
          spinosa), with blackish bark, and bearing little
          black plums, which are called sloes; the sloe.
      (b) A species of Crat[ae]gus or hawthorn (Crat[ae]gus
          tomentosa). Both are used for hedges.
          [1913 Webster]

WordNet (r) 3.0 (2006):

blackthorn
    n 1: a thorny Eurasian bush with plumlike fruits [syn:
         blackthorn, sloe, Prunus spinosa]
    2: erect and almost thornless American hawthorn with somewhat
       pear-shaped berries [syn: blackthorn, pear haw, pear
       hawthorn, Crataegus calpodendron, Crataegus tomentosa]