"On the other hand, triggers could be used against tables which do not have blob fields. I use it.  it is one of the best methods to add audit component against, existing table."

I just re-read your post, and I think I understand you point a bit better.  I think it is reasonable to write audits into a text/ntext field, although I have not seen anyone else doing that.  You could write into the same record or into a blob in another audit table (This may be easier and more flexible).  However, in either case, you should be able to write to the blob field inside an "Instead Of" trigger. 

In response to the previous post by Serqiy, this does NOT fire reentrant/recursive triggers by default.  I don't know what will happen if you turn on the recursive_triggers option - but I would not want to try it.)