The following code can be added to your WordPress functions.php file or added as a plugin to allow you to mark sections of  your post content as “Spoilers”:

function spoiler_shortcode($atts, $content = null) {
    // Extract shortcode attributes
    $atts = shortcode_atts(
        array(
            'title' => 'Spoiler',
        ),
        $atts,
        'spoiler'
    );

    // Generate a unique identifier for this spoiler
    $spoiler_id = uniqid('spoiler_');

    // Output the HTML structure for the spoiler
    $output = '<div class="spoiler" id="' . esc_attr($spoiler_id) . '">';
    $output .= '<div class="spoiler-title">' . esc_html($atts['title']) . '</div>';
    $output .= '<div class="spoiler-content">' . wp_kses_post($content) . '</div>';
    $output .= '</div>';

    // Enqueue the necessary scripts
    add_action('wp_footer', 'spoiler_enqueue_scripts');

    return $output;
}

function spoiler_enqueue_scripts() {
    ?>
    <script>
        document.addEventListener('DOMContentLoaded', function () {
            var spoilers = document.querySelectorAll('.spoiler');

            spoilers.forEach(function (spoiler) {
                spoiler.addEventListener('click', function () {
                    spoiler.classList.toggle('revealed');
                });
            });
        });
    </script>

    <style>
        .spoiler {
            border: 1px solid #000;
            padding: 10px;
            margin-bottom: 10px;
            cursor: pointer;
        }

        .spoiler-title {
            font-weight: bold;
            margin-bottom: 5px;
        }

        .spoiler-content {
            filter: blur(5px);
            transition: filter 0.3s ease;
        }

        .spoiler.revealed .spoiler-content {
            filter: none;
        }
    </style>
    <?php
}

// Register the shortcode
add_shortcode('spoiler', 'spoiler_shortcode');

Usage:

[spoiler title="Spoiler Title"]Spoiler Text here - this will be blurred until it's clicked on[/spoiler]