It would help to have more information as to where you want to go in Venice because there is no such thing as a Blue Line water taxi, or a Palazzo terminal.

There are two vaporetto companies; ACTV and Alilaguna. Alilaguna has a blue line. However, a vaporetto is not a taxi, it's a large public water bus. A water taxi is a small, private boat that carries 1-10 people. They are not associated with a color. There's a terminal for cruise ships, a train terminal, and a bus terminal, but no Palazzo terminal.

To get from Treviso airport to Venice the simplest way would be a 30-35 minute cab ride from the airport to Piazzale Roma, which is the only place you can drive to in Venice. Once you get to Piazzale Roma just walk to the vaporetto stop and take the appropriate water bus to your destination, or take a water taxi. The stands for both are right nearby. Note, the blue vaporetto line doesn't stop at Piazzale Roma. The taxi will take you close to your hotel, or to your hotel door if it has a water port.

Beware that a vaporetto is easy to board and exit, but you need a certain amount of mobility to get in and out of a small boat like a water taxi. It can be tricky.

I can't think of any way to get from the Treviso airport that connects to the Alilaguna blue, unless you took a transfer from the Treviso airport to Marco Polo airport where there is a blue line stop, then took the blue line from there.

Venice can be tough on people with mobility issues because of all of the bridges. Below is a link to Accessible Venice. It has a downloadable map that summarizes practical information to help those with impaired mobility get around the city. http://www.comune.venezia.it/flex/cm.../IDPagina/1318