the second participant will not be blacklisted

$client = new \org\jsonrpcphp\JsonRPCClient( 'http://limesurvey-dashboard.localhost/index.php/admin/remotecontrol' );
$sessionKey= $client->get_session_key('username', 'password');

$participants = array(
    array(
        'firstname' => 'firstname 1',
        'lastname' => 'lastname 1',
        'email' => 'firstname1.lastname1@test.com'
    ),
    array(
        'firstname' => 'firstname 2',
        'lastname' => 'lastname 2',
        'email' => 'firstname2.lastname2@test.com',
        'blacklisted' => 'Y'
    )
);

$result = $client->cpd_importParticipants($sessionKey, $participants);
var_dump($result); exit;

a fix could be application/helpers/remotecontrol/remotecontrol_handle.php line 3145

'blacklisted' => (isset($aParticipantData['blacklisted']) && $aParticipantData['blacklisted'] === 'Y') ? $aParticipantData['blacklisted'] : 'N'