There isn't a "secret blocked path" per se, but rather a hidden passageway that leads to a secret area. Here's how to access it:
1. Find the three Great Fairy Fountains: You need to complete the trials for all three Great Fairy Fountains:
- Great Fairy Cotera (Faron Forest): Requires finding 10 fairies.
- Great Fairy Miph (Eldin Region): Requires finding 10 fairies.
- Great Fairy Tera (Gerudo Highlands): Requires finding 10 fairies.
2. Return to the Pinwheel Forest: After completing all three trials, travel back to the Pinwheel Forest.
3. Activate the "hidden path": Once you have completed all three Great Fairy trials, a small area to the southwest of the main forest area will begin to glow with a faint light. This indicates the hidden path is now accessible.
4. Enter the hidden path: Use Magnesis to lift the two large metal squares that were blocking the passage. You will now be able to walk through.
5. Explore the secret area: This area contains a powerful enemy and some useful resources, including a chest with an ancient core that can be used to upgrade your armor.
Let me know if you have any other questions about finding secrets in Breath of the Wild!