how to get value of a select box in custom widget wordpress

You missed the ending curly brace of the class techno_widget, you need to call the function techno_load_widget outside of the class techno_widget and you declared the variable '$blog-titlewrong. You can't declare variable with-. It is core PHP` convention. So I rewrite your code like below-

class techno_widget extends WP_Widget {

    function __construct() {
        parent::__construct(

            'techno_widget',

            __('Recent Full Post', 'techno_widget_domain'),

            array( 'description' => __( 'A full post will be appeared on Sidebar', 'techno_widget_domain' ), )
        );
    }

    public function widget( $args, $instance ) {
        $title = apply_filters( 'widget_title', $instance['title'] );
        $blog_title = $instance['blog_title'];

        echo $args['before_widget'];
        if ( ! empty( $title ) )
            echo $args['before_title'] . $title . $args['after_title'];

        echo $instance['blog_title'];

        echo $args['after_widget'];
    }

    public function form( $instance ) {
        if ( isset( $instance[ 'title' ] ) ) {
            $title = $instance[ 'title' ];
        }
        else {
            $title = __( 'New title', 'wpb_widget_domain' );
        }

        $blog_title = $instance[ 'blog_title' ];

        ?>
        <p>
            <label for="<?php echo $this->get_field_id( 'blog_title' ); ?>"><?php  _e( 'Select Title:' ); ?></label>
            <select class="widefat" id="<?php echo $this->get_field_id( 'blog_title' ); ?>" name="<?php echo $this->get_field_name( 'blog_title' ); ?>">

                <?php

                $fullpost = new WP_Query(array(
                    'post_type' => 'post',
                ));

                if($fullpost->have_posts()): while($fullpost->have_posts()): $fullpost->the_post(); ?>

                    <option value="<?php the_title();?>"><?php the_title();?></option>

                <?php endwhile; endif;?>

            </select>
        </p>
        <p>
            <label for="<?php echo $this->get_field_id( 'title' ); ?>"><?php _e( 'Title:' ); ?></label>
            <input class="widefat" id="<?php echo $this->get_field_id( 'title' ); ?>" name="<?php echo $this->get_field_name( 'title' ); ?>" type="text" value="<?php echo esc_attr( $title ); ?>" />
        </p>
        <?php
    }

    public function update( $new_instance, $old_instance ) {
        $instance = array();
        $instance['title'] = ( ! empty( $new_instance['title'] ) ) ? strip_tags( $new_instance['title'] ) : '';
        return $instance;
    }
}
function techno_load_widget() {
    register_widget( 'techno_widget' );
}

add_action( 'widgets_init', 'techno_load_widget' );

And that was working. Here is the screenshot-

Now you can modify this code as you wish. But I think where you used $instance['blog-title'](in my code that is $instance['blog_title']) there may be you could use $instance['title'].

And you can get the input as getting input form any other input fields. For further information please google it.