Why is atropine a CNS stimulant, although it blocks the muscarinic receptors in the brain?
Short answer
Atropine inhibits an inhibitor and hence its effects are excitatory. 

Background
According to the wiki page on muscarine M2 receptor:


  M2 muscarinic receptors act via a Gi type receptor, which causes a
  decrease in cAMP in the cell, generally leading to inhibitory-type
  effects. 


Atropine is an antimuscarinic (anticholinergic) drug.According to the wiki page on atropine:


  [R]eactions to atropine include ventricular fibrillation, supraventricular or ventricular tachycardia, dizziness, nausea, blurred vision, loss of balance, dilated pupils, photophobia, dry mouth and potentially extreme confusion, deliriant hallucinations, and excitation especially among the elderly.


So atropine blocks inhibitory muscarinic receptors, and hence causes excitation.