What is "Block burn" and how is it calculated?
I have no idea and no connection with the aviation industry so I looked it up. I found

Block Fuel / Ramp Fuel / Total Fuel On Board
Block fuel is the total fuel required for the flight and is the sum of the Taxi fuel, the Trip fuel, the Contingency fuel, the Alternate fuel, the Final Reserve fuel, the Additional fuel and any Extra fuel carried. Source: Skybrary.aero.

That in turn led me to look up 'aviation block' and I found

Perhaps you’ve noticed that flights flown on different airlines between the same airports take different amounts of time. You might reasonably ask: why would different flights, departing from and arriving to the same airport take different amounts of time? The total amount of time a flight takes — from pushing back from the departure gate (“off-blocks”), to arriving at the destination gate (“on-blocks”) — is called “block time”, and airline block times vary for the same routes. Source: FlightStats.

I guess that block burn is the amount of fuel burnt between the departure gate and the destination gate.

My second question is how is it calculated?

Start fuel - end fuel, I guess. This can then be tabulated and used for future flight block burn calculations.

I have also heard it depends on seasonal data.

Many operational factors will vary by season so this seems reasonable.