There are a number of considerations that one should keep in mind when planning to apply plaster to blueboard. Many older homes originally had plaster walls, which provided a durable and beautiful finish to the wall. This was typically applied to a wooden lathe underlay, which was thin wooden strips applied to the wall frame, and filled in with a course sand based plaster or "brown layer" as the primary layer. Then additional layers were added, the scratch layer and finally the veneer layer. With the advent of drywall and mud as a finishing option, this gave finishers a much faster and less labor intensive option to quickly put up a finished wall. This finishing option did not give as durable of a finish though, and was not nearly as durable for cleaning or wear and tear.
Along with drywall, special "blueboard" was created that was particularly designed to hold up to the higher moisture content of a plaster finish. While blueboard has the same makeup as drywall in the core, it is finished with a thicker blue paper that can accommodate the wet plaster. Originally strips of blueboard were used, with the traditional three layer plaster system as the finish. As the process evolved however, soon entire sheets of blueboard were created to be used as the underlay, eliminating the need for the brown layer, and needed only the last one or two steps or veneers as a finish.
In applying a plaster veneer to blueboard, there is a very specific technique that is used by professionals that can be difficult to master. Applying a plaster veneer takes a knowledge of the material and being able to work on the surface quickly and competently. When the plaster is mixed with water, a chemical reaction occurs immediately and the plaster will begin to crystallize and set up very quickly. It is only workable for about half an hour before it is not able to be applied to a surface.
When working with plaster, it is prudent to mix in small batches, and apply it quickly. Typically once the plaster is mixed, trowels are used to spread the plaster onto the underlay, and trowels and floats are used to give a polished, smooth, and even finish to the plaster. A skilled plasterer can finish the plaster to a glass like finish. While plaster can certainly be painted, there are also tints and colors that can be mixed into the plaster directly to finish the surface in the desired color. Many prefer to keep the original color, a mottled cream, instead of coloring it as it blends beautifully with pretty much any decor.
If you're considering applying a plaster veneer yourself, it will be entirely worth your time to work on a sample piece of blueboard to work on your technique and familiarize yourself with the material. Mix a small batch of water and plaster according to the package directions. Using a trowel, spread the plaster onto the blueboard, and work it out to a thickness of of about one quarter to one eighth of an inch. Quickly grab a float, and holding the float at a very small angle (the float will need to be almost flat against the plaster), use the float to smooth out any ridges or uneven places. Apply a second coat in the same way, and allow to dry completely (about 48 hours).