Click on the Comments User in the New Window

Login => Layout => Edit HTML => (tick on) Expand Widget Templates => find out to see like this:

<b:if cond='data:comment.authorUrl'>
<a expr:href='data:comment.authorUrl' rel='nofollow' ><data:comment.author/></a>
<b:else/>
<data:comment.author/>
</b:if>

Change from rel='nofollow' to target='_blank'

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Done!