There are no sources for eating in the rain. It's a hergesh that chassidim are not mitztaer or won't let physical things make them mitztaer from a mitzvah.
However making a beracha im such a case is highly debatable even amongst chabad.
The rebbe made a beracha in such situations, but many chassidim feel that tzaddikim are different as they truely are not mitztaer. We just try to emulate them.