Blue toner chemically creates the equivalent of the "dye" Prussian Blue, which was one the first synthetic pigments developed sometime during the 18th century and since then a popular blue pigment for creating oil paints. It is quite stable in these paintings. The pigment created is also equivalent to cyanotypes, and they have lasted for over 1.5 century, since the dawn of photography.
Yes, they can be faded through exposure to light and alkaline (opposite of acid) conditions, but this fading is to quite an extent reversible, if the print is stored for some time in the dark, see also Mike Ware's documentations on alternative processes:
http://www.mikeware.co.uk/mikeware/conservation.html
If you start to use blue toner, one big tip is to acidify your print before toning, getting rid of the alkaline buffer in most papers, by for example putting it in an acidic bath similar to your stop bath used during development for some time (but don't use your developer contaminated stop bath for it!)