您无法在Blog小部件外部检查data:post.labels，但好消息是您可以使用Blog小部件中的代码通过JavaScript更改页面标题.

You can not check data:post.labels outside the Blog widget but the good news is that you can use your code inside Blog widget to change the page title via JavaScript.

<b:if cond='data:post.labels any (label => label.name == "LABEL1")'>
    <script type="text/javascript">
        document.title="1111 - Webitename";
    </script>
<b:else/>
    <script type="text/javascript">
        document.title="2222 - Webitename";
    </script>
</b:if>

但是，通过JavaScript更改页面标题不会帮助SEO，因为某些Web爬网程序不支持JavaScript. (Google的)

But changing the page title via JavaScript won't help SEO because some web crawlers do not support JavaScript. (Google's does)