How to insert icon in site title

That setting is html escaped, it means that all the html special chars are converted in the realated html entities, so e.g. < became &lt;.

To print correctly you can use echo html_entity_decode( get_option('blogname') );

However be aware, that blogname option is used by WordPress and by a lot WP themes and plugins (e.g. SEO plugins) that aspect that options containt text without any html. For this reason, maybe, is not a good idea insert html code in that option.