Characters on "blocked" worlds can be transfered to any world following the transfering rules like:
Optional PVP -> Optional PVP
Open / Retro Open -> Open PVP / Retro Open PVP / Optional PVP
Retro Hardcore -> Any World.
There is no transfering restriction (to outside of the world) for being a blocked world.
The main issue about getting a character on a blocked world and transfering it, its that most characters on a blocked world have higher prices, since most of the blocked world have its own economy, tibia coin price, etc. before opening it.
(Don't confuse blocked world, with locked worlds (Zuna / Zunera)... You can transfer to the locked world, but once its there you can't take the character out.)