If a lead in an import file has a blank value for a column (e.g. first name), will Marketo overwrite the existing data in Marketo when the file is imported?
Marketo does not overwrite the existing data if a field for a lead in an import file is blank.
Per the article: https://docs.marketo.com/display/public/DOCS/Clear+Field+Values
"Leaving the new value blank or simply entering a SPACE will not truly empty the field. You have to type in NULL. Also, remember, flow steps can't be undone after running."
While the example shown in the article is with a smart campaign flow step, the NULL value is also applicable to list import.