WordPress at a glance
filter is not described

pre_get_blogs_of_user filter-hook . WP 4.6.0

Filters the list of a user's sites before it is populated.

Passing a non-null value to the filter will effectively short circuit get_blogs_of_user(), returning that value instead.

Usage

add_filter( 'pre_get_blogs_of_user', 'filter_function_name_7752', 10, 3 );
function filter_function_name_7752( $sites, $user_id, $all ){
	// filter...

	return $sites;
}
$sites(null/array)
An array of site objects of which the user is a member.
$user_id(int)
User ID.
$all(true/false)
Whether the returned array should contain all sites, including those marked 'deleted', 'archived', or 'spam'.
Default: false

Where the hook is called

get_blogs_of_user()
wp-includes/user.php 602
$sites = apply_filters( 'pre_get_blogs_of_user', null, $user_id, $all );

Where the hook is used (in WP core)

Does not used.