Gutenberg; Rich Text/HTML for Metadata Textarea Control in Back End

Here’s an easy way to achieve what you wanted:

From the edit callback, return a class which extends the ClassicEdit component used with the Classic editor block. And in your block’s attributes, make sure content is set (in the question, it’s blockValue). Tried and tested working properly:

var el = wp.element.createElement;
var ClassicEdit; // An instance of the private ClassicEdit class.

wp.blocks.registerBlockType( 'my-gutenberg/my-block', {
    attributes: {
        content: {
            type: 'string',
            source: 'meta',
            meta: 'the_meta'
        }
    },
    edit: function( props ) {
        if ( ! ClassicEdit ) {
            var block = wp.blocks.getBlockType( 'core/freeform' );
            ClassicEdit = ( class extends block.edit {
                componentDidMount() {
                    // Call the parent method to setup TinyMCE, etc.
                    block.edit.prototype.componentDidMount.call( this );

                    // Change the toolbar's title - to your block's.
                    jQuery( '#toolbar-' + this.props.clientId )
                        .attr( 'data-placeholder', 'My Title' );
                }
            } );
        }
        return el( ClassicEdit, props );
    },
    save: function( props ) {
    },
    ...
} );

But you’d want to copy these CSS and make sure to replace the my-guternberg/my-block with the proper block name.

For advanced coding, you could copy the component file, edit it anyway you like, and use it with your block’s edit option — edit: MyClassicEdit.

And here’s the code I used for testing.

Preview