There's no single correct answer to this question, because there's no universal standard for blog comments. If you're using a self-hosted comments plug-in in WP, for example, or any other case where comments appear within the main HTML document, then yes: you can trigger off web activities (clicks and page views) or via your back end using one of Marketo's APIs. However, if you're using a comments service like Disqus or LiveFyre, then probably not: these services usually appear in an IFRAME and offer limited real-time server-to-server notifications.
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