How does the integration work with blank values?

Mailchimp for Salesforce does not support updating fields with blank values in either direction. If you use our field mapping or Data Wizard features to update data from Salesforce to Mailchimp, any blank values will not be sent to Mailchimp. Similarly, when updating data from Mailchimp to Salesforce using field mappings, blank values from Mailchimp will not overwrite existing values in Salesforce.

This behavior is designed to ensure data integrity. As a result, the integration will never remove an existing field value. If a field is mapped and contains a value, that value will remain intact even if the corresponding field in the other system is blank.

This ensures that no critical data is inadvertently erased during the sync in either direction. 

For Salesforce to Mailchimp imports via the Data Wizard, there is an easy workaround: 

• Create a formula on your source object (contact, lead, etc) that returns "None" or similar if the field you are trying to work with is blank. It should also return the field's value if it is not blank. Here is a sample for a text field. Picklists and other data types may require different syntax. Salesforce help
  

  IF(ISBLANK(fieldname),'None', fieldname)

• Add the formula as a column in your import source and map the formula field in the data wizard instead of the real field
• When the field is edited to blank, the wizard will push the formula field's "none" and overwrite the previous values. 

Note that the above formula may not work for field mappings. Field mappings are triggered by record edits, and a formula is not actually a record edit. Formula FAQs.