They probably wont block if you can easily cast two +4/+4 effects on your commander
Typically people pull out cards and lay them out on the tables, however I've met people who don't appreciate it, so it never hurts to establish the rules of how you guys are trading
Spoils of Evil This is another card that does a great job of making you make a bad face when you actually take it in
Its a Commander with a ton of value (Im looking to squeeze him into another underwater-themed deck as we speak), but theres also Kindred Discovery in the deck as well to get even more card draw from your Merfolk
Self-mill and graveyard decks like Sidisi, Brood Tyrant or Hogaak, Arisen Necropolis are constantly looking for more cards that can provide value from the graveyard, so this fits the bill perfectly
Elves Plays like: Slivers, Elementals, Pirates Elves are perhaps the most successful and powerful typal deck across Magic history, with a consistent presence in the metagame of tournament formats