Sometimes we see this problem with OS X 10.8 Mountain Lion.

Try this fix...
  1. Click on the blackVPN/Tunnelblick icon
  2. Select VPN Details...
  3. On the left-hand side select the VPN configuration to change.
  4. On the right-hand side select the Settings tab
  5. Change the "Set DNS/WINS" option and click Connect.
  6. Try with all different options except for "Do not set nameserver".