Are you perhaps thinking of something else?
Here are some possibilities:
* Cut trees: Maybe you are referring to the trees that block your path in some forests. To clear these, you need to use the Cut HM.
* Strength: Some paths might be blocked by rocks or boulders. These can be moved using the Strength HM.
* Hidden paths: There are some hidden paths in forests, often accessed by using items like Flash or interacting with specific objects.
Please clarify what you mean by "forest blocks" and I'll be happy to help you find a solution.