I can export Razor TBBs just fine from my Tridion 2013 SP1 system (using Content Porter 2013 SP1), however whenever I import, it skips them - they are not counted in the successful or failed items in the summary at the end, or listed in the list of processed items, and there are no errors in the logs either the Content Porter log, or the Tridion Event log on the server. I have tried importing on a separate environment, but also importing on the environment that I originally exported. The behavior is the same on both, leading me to believe that this is a Content Porter, rather than an environment/installation issue.
UPDATE: Further testing shows that this actually only happens with a Helpers TBB which is configured in the
Tridion.ContentManager.config file as automatically imported for all TBBs. If I manually create this in the target environment, subsequent TBBs (which have a dependency on this) are imported OK.