[SalesForce] Is it possible to update a boolean field within a data extension when an email is sent

I think you're looking for the _DataSourceName personalization string.

This value captures any audience object sent to. This covers lists, data extensions, groups, and filters.

Usage:

%%[

if _DataSourceName == "DE1" then

  /* something for DE1 */

else

  /* something for DE2 */ 

endif

]%%

You may need to experiment with it if your send is every sent to more than one DE.

Update 2020-07-07

You can get a bunch of info about journey interactions via REST API. It'll give you all the details about the send context and email definition, etc. Technically, you could pull this data in an Automation with a Script Activity and store it in a DE. Similar to this, except with a REST call instead of WSProxy retrieve on the SOAP object.

Let's assume the columns name is EmailOptIn

Try this:

Select
True as EmailOptIn
From DataExtensionContainingAttributes
Where
EmailOptIn = False

Update:

As Adam Spriggs suggested, you can also use 1 for true and 0 for false