I do not think you can do it with XOG alone.
The sample XML files for project do not have Blueprint tag nor do the XSD files.
What you could do is create a a custom attribute where you store the Blueprint and once XOGed in manually or with a process write the value to the real Blueprint field.
Below is an xml file which has a custom numeric field and a custom string field for the blueprint. That was obtained by reading a project out from a system where the custom fields were added and values written to the fields
Those fields and the real blueprint can be added to the edit project properties view in Classic
Note that the Blueprint Active ID is the code (string) and read only and gets the value based on the lookup used to get the value to Blueprint. Thus my numeric aBlueprintID is wrong data type and it is not even not needed. Once you have XOGed in your project with a valid value in the custom field (aBlueprint in my case) you can set the value of the OOTB Bluerint field the same as in the custom field.
You can set the value also in the New UX if you add the fields to the project grid.
If you do not want to do it manually you should be able to create a process to do that..