When you're in the Control Panel view, you have leads and blocked. What does Blocked mean?
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Blocked means that those are lead that either opted out or have bad email addresses(hard bounces)
View Blocked Leads in a Smart Campaign - Marketo Docs - Product Docs
"Blocked leads are people in the Marketo database that are marked blacklisted, unsubscribed, marketing suspended, and that have invalid/empty email."