Yes, SearchBlox does support stemming, and it can be configured on a per-collection basis if required. A stemmed search for a word finds the exact same term, as well as terms that are derived from the same meaning and part of speech as the search term. When stemming is enabled, inflected words are reduced to a root form. For example, "running", "runs", and "ran" are the inflected form of "run".
Stemming can be enabled or disabled for each collection through the Collections > Settings Admin menu, and it is disabled by default.