Blank for those data types would be null.
That said, if you're planning the 'pattern' of inserting a blank row then updating it to actual values, please reconsider. It can cause huge fragmentation problems as well as being a rather poor design pattern. Just insert the real values.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability