Target TagID from URL inside IF/Else statement

Can I not target the 'Tag ID' of a URL? below is one of the methods I've tried so far:

{% if (CMSContext.CurrentDocument.RelativeURL == "~/blog-(1)/?tagid=381") { #%}
Show this
{% } else { #%}
Show something else
{% } #%}

This just returns the 'else' result on every page. So it's not working.

@Brenden Kehren. Thank you for you're reply. I modified it to this and it works:

{% if (QueryString.GetValue("tagid") == "381") { |(identity)GlobalAdministrator%}

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You're referencing the actual URL stored in the database and not the rendered URL with the parameters. SO you might try something like:

if(QueryString.GetValue("tagid") == "381"}

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