The Collaborative International Dictionary of English v.0.48:

Block \Block\ (bl[o^]k), v. t. [imp. & p. p. Blocked
   (bl[o^]kt); p. pr. & vb. n. Blocking.] [Cf. F. bloquer, fr.
   bloc block. See Block, n.]
   1. To obstruct so as to prevent passage or progress; to
      prevent passage from, through, or into, by obstructing the
      way; -- used both of persons and things; -- often followed
      by up; as, to block up a road or harbor; to block an
      entrance.
      [1913 Webster]

            With moles . . . would block the port. --Rowe.
      [1913 Webster]

            A city . . . besieged and blocked about. --Milton.
      [1913 Webster]

   2. To secure or support by means of blocks; to secure, as two
      boards at their angles of intersection, by pieces of wood
      glued to each.
      [1913 Webster]

   3. To shape on, or stamp with, a block; as, to block a hat.
      [1913 Webster]

   4. to cause (any activity) to halt by creating an
      obstruction; as, to block a nerve impulse; to block a
      biochemical reaction with a drug.
      [PJC]

   To block out, to begin to reduce to shape; to mark out
      roughly; to lay out; to outline; as, to block out a plan.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

blocked \blocked\ adj.
   1. closed to traffic. Traffic was blocked by an overturned
      tractor-trailor

   Syn: out of use(predicate).
        [WordNet 1.5]

   2. at a complete standstill because of opposition of two
      unrelenting forces or factions.

   Syn: deadlocked, stalemated.
        [WordNet 1.5]

   3. unusable because of some obstruction.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

blocked
    adj 1: closed to traffic; "the repaving results in many blocked
           streets" [syn: blocked, out of use(p)]
    2: completely obstructed or closed off; "the storm was
       responsible for many blocked roads and bridges"; "the drain
       was plugged" [syn: blocked, plugged]