I have owned both. As of now (3) Blackbirds, (2) NX-250's and a North American.
There really isn't much to compare the two brands in my opinion. So I'll just try to give you my observations.
The Blackbird is VERY well made and very light. The sewing is so well done I posted a thread about it some time ago. It's also huge considering it's weight. You lay pretty diagonal in it. I love the Blackbird and only have two complaints that have nothing to do with functionality.
1. With "my" feet in the box I really can't see out to my left side.
2. The bird has wings, and those wings get staked out. So, when you get in and out of it, the shock cord is all in your face.
That's it in the complaint department. Nothing big as you can see. When you lay in it, it's soft and comfortable. It packs small. It also looks pretty cool.
The Clarks are totally different really. The lay is less diagonal. Which I prefer. The NX models have poles which get on my nerves. They are not shock corded, and there isn't an obvious place to pack them. The NA requires you to somehow tie up the weather shield and bug net. It's not as wide as the NX models or as long. They also have pockets underneath that Clark claims help keep you warm. Personally I just don't see it. They are nice in the summer, but I could do without them.
The Clark weather shield: this is huge for me. I simply love this feature. It does add warmth, no doubt about. It also adds a certain bit of privacy.
*There is an aftermarket solution which you can buy from AHE for the Blackbird.
Gripes about the Clark.
1. It's heavy for a hammock, this is somewhat negated by the feature set. Never the less, it's heavy.
2. Clark doesn't cut their material with a hot knife or something. After a while you start to see little whiskers from the material. This hasn't ever been a problem, but it just seems like an overlooked detail for such a high price item.
3. Tarp seems small. It make no difference to me, but it puts some people off.
Right now I choose a Clark. Only because the lay of it suits me right now. There is absolutely nothing wrong with either one of them and they are both great.
Don't get caught up in the hype of either one in my opinion. The Clark costs a lot and it comes with a lot. They also make a lot of claims that don't amount to a lot if you ask me. The Blackbird has a cult following and there are some zealots out there, but it's well deserved.
In my opinion, They are the two best hammocks, but not because they are like each other. It's because they both stand and deliver. Hope that helps.