I want to develop a custom theme for Magento 2, but I can't find the default blank theme of Magento. The folder "/app/design/frontend/Magento" is empty. Shouldn't there be a "blank" directory? Can't find all the default templates...
I think you downloaded and install Magento2 version from https://www.magentocommerce.com/download.
You can find it in
To answer your comment about why the folder location of the themes differ, I will reference a post I found.
When installing via Composer (as recommended for production sites or anyone else wanting stability), Composer puts it under the vendor directory. This is standard Composer behavior. So "vendor/Magento/theme-frontend-blank" is the root directory of the Composer package holding the blank theme. Magento will find the theme over in that directory as well. (This is one of the reasons there is a "registration.php" file in themes (and modules) now. It tells Magento where the theme sits on disk.) So while developing a theme, or if theme is for a local project (not intended to be used across multiple projects), you put it under app/design. If you decide to sell it on Magento Marketplace for example, there are instructions to bundle it (basically ZIP the directory) and upload to the Marketplace. Other users that download it will have it put in their 'vendor' directory automatically by Composer. The 'vendor' directory holds all code you did not write.
Reference Link: https://github.com/magento/magento2/issues/2577