That wood block certainly is not supporting much of anything. My guess is it was toenailed there to hold the joist in place until it was supported at the ends and was never removed. (There are posts supporting the end opposite the house, right?)

Are those 2X4 joists? If so, you might want to check the span tables to see if they are overspanned.

How is the deck supported at the house? Ledger bolted to the rim joist?

As far as flashing, it should go on top of the deck joists and behind the siding. It's purpose is to keep water from getting between the rim joist and ledger board and rotting them out.

Was this a Harry Homeowner-built deck?

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