There is no easy way to add a logo to a space blueprint. The variables you can add in the UI are limited.
Per Customizing Default Space Content:
The following variables are available to be added to the default space content templates.
$spaceKey - inserts the space key into the site space homepage
$spaceName - inserts the space name into the site space homepage
$userFullName - inserts the user (owner of the personal space) into the personal space homepage
$userEmail - inserts the email address of the user (owner of the personal space) into the personal space homepage.
If you have development skills, this guide may be of help:
Write a simple Confluence Space Blueprint
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