Pretty simple question: can stainless steel guns be blued?

Thanks.

No, but there are other ways to blacken stainless.

As already answered, generally no.

Supposedly, SS can be plated with iron and then blued in the traditional ways.  At that point, the surface is just as rust prone as any other blued gun.

Brownells claims to have a bluing mix for stainless.

There is plenty of iron in stainless but the surface atoms are normally Chromium.

To the point that passivation using strong acids is used to remove any exposed iron atoms on the surface.

LINK

As above, Brownell's sell a hot salts chemical that "can" blue stainless steel.
There were several refinishing companies that offered the service.

However, the color wasn't as dark or as durable as people wanted, and the out-come was "iffy", needing a lot of experimentation to get it right, along with all-to frequent failures needing another try, or tries,  to get it right.
Due to the difficulties with it, everyone I know of stopped offering it.

There are processes that will blacken stainless, but not too many companies offer them.
One, which is not very durable is black chrome.  
This is less durable then standard bright nickel.  Again, most companies stopped offering it due to customer complaints about lack of durability.

Sig's "Nitron" is blackened stainless, no?

Smith and Wesson has a nice black finish on the mp pro series stainless slides

Korth used PVD, the same finish as on Luminox stainless watches. Physical vapour deposit is available in different colours.