The Vanish Cap in Super Mario 64 turns Mario invisible for about 20 seconds and while this power is activated, it allows Mario to walk through certain objects such as metal fences, walls, and enemies.

The first step that you need to do in Super Mario 64 is to gain access to the basement in the castle, which can be done after defeating Bowser in the Dark World and obtaining a special key.

Run around the basement area until you find an odd wooden door that seems out of place and then you’ll be taken to a small room with a pool of water. Go swimming and follow the arrows which will lead you to a room with two pillars that you’ll need to ground pound.

This will drain the water in the moat that’s outside of the castle, allowing you to freely explore the surrounding area. You’ll now find that a small square-shaped hole has appeared; fall in and you’ll be taken to the Vanish Cap Switch Course.

You’ll be given a Vanish Cap as soon as you fall into this level and you need to then make your way through the course until you end up finding a huge blue Switch near a power star.

Now all of the blue blocks that were previously unable to be activated can now be used whenever you encounter them in stages.