After you get to Blackwell's mansion, walk underneath the landing site towards the fence, towards the spot where it can be crossed and walk through the gate. After you have defeated the guards, drill a hole in asphalt surface in the marked spot, to obtain dynamite. Plant the dynamite by the statue, next to the building entrance. This will reveal a key hole.
You will find the key to the right, past the playground, in a small room. It is in the safe. Afterwards, return to the keyhole, open it with the key and enter the building. Carry the crate on the right side of the corridor out from the building. Using the crate, you will build an RC car outside, which you can use to blow up the garage door. There, cut off the power supply in the fuse box and climb to the higher floor.
Once you get there, smash the pool furniture. There is a trail there that will take you to a valve. Mount the valve to the right side of the pool, on the wall, and you will be able to drain the water. Thanks to this, you will find a color gun swapper at the bottom of the pool and deactivate electricity on the abseiling wall.
Climb up and go right. Water the plants and jump over them to a higher level. There, remove the stones and fix the fuses. Below, the hidden door will appear. Cross the door to enter the mansion.
Examine the traces first (1) (there is a hidden one behind a piece of furniture on the right (2)). This will lead you to two books thanks to which you will open a secret terminal. In turn, it will open a hidden safe with dynamite inside. Plant dynamite by one of the statues in the center (3) to perform actions required to end the level.