Intercept Ninja Forms email to remove the BCC option from email headers and send theme separately.

nf-filter-email-bcc.php

<?php
/*
Plugin Name: Ninja Forms Custom email send
Description: Testing Ninja Forms email hook to catch bcc and send them separately
Version: 1.0.0
*/

add_filter( 'ninja_forms_action_email_send', 'nf_custom_email_send', 10, 5 );

function nf_custom_email_send( $return, $action_settings, $message, $headers, $attachments ) {

    //Intercept precise email action, EDIT 21 to the precise action ID
    if( $action_settings['id'] === 21 ) {

        $recipients = [$action_settings['to']];

        foreach( $headers as $key => $header_line ) {
            if( strpos( $header_line, 'Bcc:' ) === 0 ) {
                $bccs = explode( ',', $action_settings['bcc'] );
                foreach( $bccs as $bcc ) {
                    $recipients[] =  $bcc;
                }
                unset($headers[$key]); 
            }
        }

        foreach( $recipients as $recipient ) {
            $sent = wp_mail($recipient, strip_tags($action_settings['email_subject']), $message, $headers, $attachments);
            if (!$sent) {
                $return = false;
                break; 
            }
        }

    }

    return $return;
}