Delver of Secrets is significantly less prevalent in Block Constructed than in Standard, with most Delver decks running it alongside Invisible Stalker with cheap buffs like Furor of the Bitten and Spectral Flight or the occasional equipment package to turn Stalker into a real clock. Red Blue and White Blue seem to be Delver's home in Block, with Red Blue featuring a more Aura-oriented buff suite, as compared to more Equipment in the White version. Both decks can be incredibly powerful, though, so don't let the fact that they're less popular than RW fool you into showing up unprepared. Latch Seeker seems like a potential fit here, given his unblockable status, though the lack of Hexproof does make him less appealing than Stalker. Tandem Lookout's Soulbond, pairing it with a Stalker or other evasive creature seems like another approach, garnering some card advantage.
Finally, Blue Green sees its share of play, working on the self-mill plan to fill up the Graveyard a la Armored Skaab. Once there are plenty of creatures in the yard, Splinterfright can be quite a beating, especially alongside Kessig Cagebreakers. I'm less impressed with Ghoultree in general, but I guess it's hard to go wrong with a 10/10 for just a couple mana, which seems easy enough to accomplish with these builds. Gnaw to the Bone helps offset the aggro advantage with lifegain. Mulch helps smooth out your mana and fills up your Graveyard, while Tracker's Instinct gets you your choice of creatures from the top four cards, helping to dig up the finisher of choice for the situation. Unfortunately, Avacyn Restored has done away with a lot of the Graveyard shenanigans, so I'm at a bit of a loss as to how this deck will fare once it hits. It could potentially be tuned to work with Descendant's Path, given the Human and Zombie elements of the creature base.
Ultimately, if you're looking to participate in a Block Constructed event, either in paper or online, I would strongly urge you to make very certain that you've got a strong Aggro matchup. The format is aggressive right now but with that comes opportunities for anti-aggro builds to shine. Sleeve up your Blasphemous Acts and watch out for Invisible Stalker.
Q: I sit at 6 life and control Teysa, Orzhov Scion, three 1/1 White Spirit creature tokens with Flying, and Butcher of Malakir. My opponent controls three 6/6 Green Wurm creature tokens which are currently attacking. Before blockers are declared, I activate Teysa's first ability, sacrificing three Spirit tokens to pay its activation cost, targeting one of the Wurm tokens. In response, my opponent casts Beast Within, targeting Butcher of Malakir. What happens?
A: In short, your opponent will sacrifice all of his Wurm tokens to Butcher of Malakir's triggered abilities and Teysa's ability will be countered on resolution. Sacrificing three white creatures is the cost of activating Teysa's ability, which is done after you've announced that you are activating the ability and chosen targets. As soon as you sacrifice those creatures, Butcher's ability triggers, but is not yet put on the stack. Once you have finished paying all costs of activating Teysa's ability, namely sacrificing three White creatures, the ability is considered activated, and you would normally get priority again immediately. Since Butcher's ability has triggered three times, however, those triggered abilities must first be put on the Stack before anybody receives priority. As such, when you receive priority again, the Stack contains, from top to bottom, three instances of Butcher's triggered ability and one instance of Teysa's activated ability. When you pass priority, your opponent will have their first opportunity to cast Beast Within on your Butcher. To be sure, when they cast Beast Within on Butcher of Malakir, once that resolves, his ability will trigger again, ultimately leaving four instances of its triggered ability on the stack above Teysa's activated ability. As each resolves, the opponent must sacrifice a Wurm token, until all three have been sacrificed. The fourth Butcher ability will have no effect, and finally, Teysa's activated ability will be countered for lack of a legal target.
602.1. Activated abilities have a cost and an effect. They are written as "[Cost]: [Effect.] [Activation instructions (if any).]"
602.2. To activate an ability is to put it onto the stack and pay its costs, so that it will eventually resolve and have its effect.
601.2h Once the steps described in 601.2a-g are completed, the spell becomes cast. Any abilities that trigger when a spell is cast or put onto the stack trigger at this time. If the spell's controller had priority before casting it, he or she gets priority.
603.2. Whenever a game event or game state matches a triggered ability's trigger event, that ability automatically triggers. The ability doesn't do anything at this point.
603.3. Once an ability has triggered, its controller puts it on the stack as an object that's not a card the next time a player would receive priority.