I've been playing Mictlan and Abysia Blood of Humans in MP a lot the Last couple of months and I have to say that I prefer Mictlan a lot better as a blood nation (except for the mico managing the priest blood sac every turn which makes my turns take too long for blitzes).
Mictlan has cheap blood hunters. 80 gold sacred 1 Blood priests with the diving rod are the best gold to bloodhunt ratio there is. Use them, and use them often. They are also the best researcher per gold for Mictlan. I recruit them every turn because they are so versatile. They can be built even at your non-capitol castles.
As Norfleet has said you need castles to protect your bloodhunters or they'll die easy to SC's. I've beaten big armies with just my standard bloodhunters casting Summon Imp over and over but I wouldn't rely on that. You also won't be getting much gold from your provinces since you should be bloodhunting all of the ones that are population 4000-6000, so Watchtower is the best way to go for defense.
My strat is generally to use mercenaries and a SC Pretender to expand and not the nation troops. I really really hate Mictlan's troops. I only recruit the priests and independant scouts as units. Using devils and fiends as a base army works fine because they don't cost upkeep. You won't be getting much gold, but you won't need much gold either. If you get a more expensive tower or if you recruit a lot of troops you'll be screwed due to the unrest caused by your bloodhunters.
What my core of combantants are: Fiends and Devils, along with Ice Devils, Arch Devils, and Heliopolii eventually. Maybe a few succubi if I'm feeling frisky. That's more than enough to beat most armies, even in MP. As long as you are blessed with close provinces that are 4000 Pop+ you can start the blood economy early. The priority is to build a cheap castle (preferably Watch Tower) quickly so you recruit blood priests in two provinces a turn.
Mictlan's priests are super province searchers. Water, Astral, Nature, Blood, Holy and Fire are easily searched out with them. Use that to your advantage as well. However, after I get 1-3 in some paths of magic I totally focus on Blood Magic to summon the Devil Commanders before anyone else. Keep them alive and geared up and you can corner the game with the unique Devils since nobody else can summon them once they are in the game (except with a Wish I think).
Well, that's my general strategy. I have one game where I'm blood searching 6 provinces and getting 100+ slaves a turn. I'm summoning a demon leader per turn with a High Priest with some +blood gear and I can easily empower him up if I need to summon other demons since I don't use my gems for much else aside from forging gear.
Also, Mictlan gets the Fiend summon by default so use that to your advantage. Once you get your first blood province working with 4 priests searching, summon 2-4 fiends per turn and you'll have a huge flying demon army before anyone else is at the point of summoning anything on that scale. I've wiped out huge Ulm armies with many crossbowmen and infantry easily with 20 fiends. About the only national units that can stand up to fiends that early are the Paladin, heavily armormed lanced units or possibly a bunch of fire blessed units. If you notice a fire bless coming at you, switch to building Demonic Contracts (80 slaves to summon 1 demon per turn) instead of summoning fiends. Fiends are great early on but you will want to switch to Devils later on because of the fire protection.
Sorry if this was a bit rambling
Good luck with Mictlan, it's really the best bloodhunting nation in my experience.