Here is a discussion on Blighthawk breed preferences. http://www.warcraftpets.com/community/forum/viewtopic.php?f=3&t=9543&p=76281&hilit=blighthawk#p76281
Why is it a good pet? Well Undead racial. Slicing Wind, Cyclone and Lift-Off are strong against Aquatics(often something undead are not) and with 305 Power it will hit hard. Can be used in a Clonedancer team and with Consume Corpse it will last long(just remember to sacrifice some other pet first). When I use the Blighthawk I like to use it to hunt Anubisath Idols with Infected Claw, Consume Corpse and Lift-Off. At 273 speed it is one of the fastest undead, while still being strong, and with Lift-Off it has an avoidance move which is very rare amongst the undead. I don't think I have ever used Ghostly Bite, but I have seen it effectively used in battles. Often my opponents just do Ghostly Bite and the switch, Ghostly Bite hits really hard.
Unborn Val'kyrs, there are quite alot of ways to deal with them. Easiest of course is to grab an aquatic. Emperor Crab is popular since it can both heal and totally negate Haunt with the Shell Shield. I like Frogs(just grab one that is faster than Valks), they got strong attacks vs the Valk and is aquatic(all aquatics takes about 50 damage per turn of Haunt) and it can heal. Or use a Crocodile(Snarly/Toothy), to get a bleed on the Valkyr, possibly do a Blood in the Water before the Haunt. Then when Haunt has run out the Valk will be bleeding in backline.
Otherwise you can grab a critter. Especially one with Crouch(halves damage taken) and Burrow(so you can burrow on the turn that Curse of Doom is going off to avoid the damage completely). An example of critters with both Crouch and Burrow are Marmots. Rabbits are also very nice to use with Dodge and Burrow.
Another thing you can do is use a strong pet with a Stun. For example I have used Darkmoon Monkey to pretty much take out a Valkyr before it Haunted. While the pet itself might also die you won't have to worry much about the Valkyr during the rest of the battle.
A real easy one is Voodoo Figurine or Sen'jin Fetish. Cast a DoT, or DoT plus Wild Magic. Then after the Haunt you just Sear Magic. You will have taken 150 damage while now the Valkyr will be backlined taking DoT damage.
Direhorn with Trihorn shield is anoter nice one. You can do some real damage if you get a lucky stun. Otherwise the Trihorn Shield negates alot of damage. Just make sure it will be active when Curse of Doom is about to go off.
Alot of pets can take much less damage from Haunt by using Shell Shield, Stoneskin, Jadeskin, Spiked Skin. Sandstorm can of course also be used but both the Guardling and the Idol are humanoid so not really best choice of opponents for the Valkyr. Note that Living Sandling also can cast Sandstorm.
Just a few ways to go about the Valk'yrs.