Welcome to Android Central! Are you talking about deleting the number from your Contacts app? Are you blocking the number via contacting your carrier or via your Phone app?
I would assume that if you blocked the number through your carrier and deleted it later, the contact in question would remain blocked. I'm not too sure if it would stay blocked if you had blocked and deleted the contact via your Contacts/Phone app. I believe so though, because I block spam numbers often and they are never added as contacts (so it's almost like they were "deleted") and they don't contact me again.
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