This error happens in very rare cases where, for some unknown reasons, the data was not properly recorded in the database. In Piwik, all reports data is compressed (gzip) and then stored in the tables piwik_archive_*.
To fix this issue, look at the error message and search for “piwik_archive_blob”. For example, if the error looks like:
#0 Piwik_ErrorHandler(2, gzuncompress() [function.gzuncompress]: data error, /path/piwik/core/Archive/Single.php, 250, Array ([name] => Referers_searchEngineByKeyword,[typeValue] => blob,[table] => piwik_archive_blob_2009_05,[...]
You can find “piwik_archive_blob_2009_05″. Please delete this table in your Piwik database. Piwik will process it again next time you are viewing reports. This will fix the issue.