SQL insert / update command - blank or NULL is better

if insert / update by asp.net built-in command, it automatically convert those blank textbox to NULL field

if it is done by code, then it stores as BLANK field

is NULL better than BLANK?  any considerations?

after looking into my database table, some records are blank and some records are NULL.  seem not good

if i wish to save blank fields to NULL by code, can i do it by any inline code because looping all the textboxes to create SQL command seems not easy to maintain.

Thanks a lot.