Do you mean that everyone can see the block, once it has been added, or that everyone has the ability to add the block?
The block/BLOCKNAME:addinstance capability controls who is allowed to add the block to a Moodle page.
There is no standard Moodle capability that will automatically control who will see a particular block once it has been created - you will have to write some code in your block to do this yourself.
Thankfully, it is really easy to do, just add the following to your block's 'get_content()' function:
if (!has_capability('block/BLOCKNAME:view', $this->context)) {
return null;
}
(Note: this assumes that you replace 'BLOCKNAME' with the name of your block and that you have defined the block/BLOCKNAME:view capability in access.php and, if necessary, you have bumped up the version number to install this new capability).