On a category archive page, in a filter called from a block outside a loop, I noticed that the get_post function with a null first parameter returns the first loop's post.
Is it supposed to return such a value ? I would have imagined that the global $post variable should have a value only in a loop or on a post page.
Yes, this is how it is supposed to work. The global $post variable is set quite early in the page load, at the wp hook. As you can see from the hook order, this is before the loop starts.
Basically the $post object exists independent from its actual content in the database, which you retrieve during the loop. It's specifically meant to be accessible all the time during page generation.
get_post will give you the first post of the query and get_posts all of them.
$post variable is created. Next there are filters that may change the query (for instance amount of posts on a specific category archive page). Only when all this is done the query is actually executed and content is derived from the database.