There is one field that could be truncated by the migration. The Name field form the old Blog comments was nvarchar(100) but the one in the the new comment system is nvarchar(50). I did not realize this difference and did not expect names longer than 50 chars.
You could get past this error by editing the script /Setup/applications/blog/SchemaUpgradeScripts/mssql/188.8.131.52.config
SUBSTRING(bc.[Name], 0, 50),
Hope that helps,