Accessing data through associations is fine. But for radios or checkboxes you want to do a find('list). Your model and variable name does not match the CakePHP convention, there should be no underscore.
Properly named this should be already enough to populate the input.
// controller
$this->set('bloodGroups', $this->Donor->BloodGroup->find('list'));
// view
echo $this->Form->input('blood_group_id');
If you don't follow the conventions for some reason:
echo $this->Form->input('blood_group_id', array(
'options' => $bloodGroups
));
See: