No.  Posting an original idea in a comment on a blog entry is not good for you for two reasons:

1. iMechanica gives the time for initial posting of a comment, but does not give a new time when the comment is revised.  So, you cannot establish the priority of the idea posted in the comment.  By contrast, each revision of a blog entry is recorded, with its own time stamp.
2. It appears search engines are less diligent in indexing comments as they are in indexing blog entries.

While both reasons are technical and changeable, we have not found ways to change them yet.  So, it is better for you to post your original ideas as blog entries. 

If you find an original idea relevant to blog entry A, you can make a comment beneath entry A with a link pointing toward blog entry B that contains the original idea.