I went from an HH ULB to the Blackbird. Some differences between my two hammocks that make me happy:
The fabric is different. I like the 1.1 sil better than 70D nylon taffeta. The ULB weighs in at 1 lb 15 oz, or 31 oz. The Blackbird (double layer, 1.1 sil, ie my version) weighs in at 24 oz. plus 4.5 oz for the line/strap suspension. The sil feels better to me.
The HH bottom entry didn't bug me much, but sometimes it was a nuisance, particularly when I hung lower for whatever reason; we don't always get optimum tree spacing or the absolute best location. The Blackbird is easier to get out of when hung low to the ground. I have also found random stuff (gloves, socks, hat) that sometimes ends up floating around in the hammock with me will wind up on the ground underneath the slit on the HH. That stuff goes in the shelf in the BB. I suspect that were I to use the hammock as a bug bivy on the ground, I would be happier with the zipper than the slit. Once in a while, rolling around in my sleep, I would put my foot through the slit and wake up abruptly with a cold spot under my legs and the velcro stuck to my sock.
Put a quilt on the HH, I can't sit in it like a chair without worrying about the quilt - you have to fold it over on itself and lean against the bugnet. With the full length zip this is not a problem on the Blackbird, nor do you end up with your head in the netting. All I worry about then is sitting so low that the quilt brushes the ground.
The netting flips out of the way. It's also easy to fasten out of the way.
The tie outs are adjustable with cord locks and have reflective cord. The HH tie outs are black, you must move the stakes to adjust, and in the dark the shock cord is a pain in the shins.
The HH is 100 x 64. The Blackbird is 101 x 72. (101 is ridgeline length; the hammock is 120 total according to Brandon's site.) The Blackbird feels bigger/wider (feels bigger than 72", if you ask me) and the bugnet does not lay as low overhead as my HH net did.
Hennessy claims the ULB is for folks up to six feet tall. I'm 5'6" and I find it on the cramped side. The Blackbird doesn't feel too short.
that spot that gets uncomfortable under your calves - very prominent in the HH, not really noticeable in the BB until you try to lie end to end instead of on the diagonal.
To adjust the quilt or anything else on the HH, you get in and out a lot of times. to adjust anything on the BB, you reach out and do it.
Overall, I can tell the BB is put together very well. At times I did things in the HH like push myself off the pad to roll over (no double layer, polypro stuck to my pad!) and heard seams creak. I am a very active sleeper and that made me nervous. The BB doesn't appear to have limits so far as what position you're in. Sitting or lying down with the bugnet wide open, putting my feet to the left instead of in the footbox, getting into my usual semi-fetal back and side sleeping half twist - whatever, I don't notice any seam stress and have yet to hear it complain. I like that I don't have to sleep in one direction all the time.
I never liked the pockets on the ridgeline. The shelf is better. I don't smack my head on something in the morning before coffee.
The quilts I have were easier to adjust on the BB. They were a pain on the HH. Which is funny, as JRB seems to make them for HH users. I kept having cold spots on the HH - on the BB it took one adjustment to get it right. I could feel around between and under the hammock, reach up and tighten the head end without getting out, and it took moving the prussick from the strap to the line above the knot... and it was warmth and bliss, all night long, except for when I rolled over in my sleep and knocked the top quilt askew.
I much prefer the strap and tri rings to the Hennessy suspension. While it's certainly possible to DIY different suspension or add a zipper to the HH, I didn't feel sufficiently capable. I was thinking of getting a zipper mod when the BB started to appear on the forums. When people mentioned how roomy it was, my claustrophobia started rationalizing the purchase.
I can certainly appreciate your confusion about the differences, but those subtle little differences make the experience totally enjoyable, versus mostly enjoyable. The last thing - Brandon will do custom work within limits. If I wanted the footbox on the other side, he'd very likely have done that. Not many mfgers will do that. Brandon also spent time answering questions in email, and he makes them right here in the US. All of this adds up to a package I can endorse without reservation.