The hull is blacked to protect the section of the boat that is in the water and just above the water line. The process protects the steel in the hull from rust, pitting, and the unavoidable rubbing that occurs when you enter/exit a lock, come in contact with something submerged in the water, get stuck in the mud, come in contact with the sides of the canal and such. By periodically blacking the boat you can extend the life of the boat.
The boat needs to be taken out of the water for the blacking to be done. Ours will be floated into a dry dock and the water will then be removed so that the work can be done.
Blacking is a three day process at a minimum. Where we are having ours done it is a six day process so that complete drying is assured between the three parts of the process.On day one the Boat will be pressure washed to remove as much of the old blacking and gunk (a technical term) like weeds, mud, and rust as possible. Doing this prepares the hull for the new blacking to be applied. Day two will be a "rest" day to make sure the boat is completely dry before the re-blacking is done.
In two to three years, repeat. :-)