Use Blogger Post Label as Meta Keyword : Practically Done and Proved

Search for  <b:if cond='data:post.labels'>
and add below line after first </b:loop>

The Line is

&lt;meta name=&quot;keywords&quot; content=&quot;<b:loop values='data:post.labels' var='label'><data:label.name/><b:if cond='data:label.isLast != &quot;true&quot;'>,</b:if></b:loop>&quot; /&gt;

And Save the Template.. You are Done

See the Demo Here