Hidden Blocks can only be activated when Mario is jumping and hits the block from below. A Hidden Block will not be activated if Mario passes through it without jumping, such as: uncrouching, climbing a vine or riding a platform.
If Mario is Super Mario or higher he is able to start ducking and jump before his hitbox is lower than the hidden blocks allowing him to perform a duck jump when hidden blocks are at head level.
Ducking Wall Jump
Note: This method only works on levels created before version 1.46.
When Mario duck jumps and enters a wall slide his head will become higher than the bottom of the Hidden Block allowing him to pass through it.
Note: This method only works before version ?.
Collecting a falling power-up in midair will cause Mario's hitbox to grow allowing him to pass through the bottom of the Hidden Block without activating it.
Lakitu Cloud / Clown Car
Crouching just before touching the hidden block will allow Mario's head to be above the hidden block when he uncrouches.
Duck jumping as Fire Mario, Cape Mario or Raccoon Mario and shooting a fireball or doing a cape spin/tail whip just before activating the hidden blocks will allow Mario to pass through without hitting them.
When Propeller Mario is crouching and starts to propel upwards he will immediately uncrouch, allowing Mario's head to pass through the Hidden Block before he starts traveling upwards.
Shell on Block
When a block is hit by a shell it will slightly move upwards. This causes Mario's head to be above the bottom of the Hidden Block allowing him to pass through it.
Star Wall Jump
When Mario has a Super Star, he is able to wall jump through Hidden Blocks if he jumps off the wall while being directly under it.
If Mario Spin Jumps on a Thwomp as soon as it starts to raise he is able to pass through Hidden Blocks.