http://vido.com.ua/upload/uploaded_images/28500/28757img_7677.jpg
This is the rear of a Gen8 HP blade. See that big rectangular connector dead centre with lots of little holes? That's where everything on the blade connects to the midplane of the chassis. The bigger holes directly to either side are for power.
http://vido.com.ua/upload/uploaded_images/28500/28758img_7679.jpg
This is the front. The only thing you can hot-replace are the two hard drives (this particular photo has blanks instead of actual drives). For anything else, you have to pull the blade out of the chassis (instantly powering it off).
http://vido.com.ua/upload/uploaded_images/28500/28754img_7674.jpg
This is the inside of the blade; the front of the blade is on the left, the rear of the blade is on the right. See on the right side (the rear), there are two dark-grey rectangles with lots of pins? Those are for plugging cards in (called mezzanine cards). You can have two cards, one with two ports, and one with 4 ports, and then the motherboard has 2 ports on board. So there are a total of 8 ports, which map to 8 slots at the rear of the blade chassis, like this:
1 2
3 4
5 6
7 8
http://h30499.www3.hp.com/t5/image/...9D160DAB86/image-size/original?v=mpbl-1&px=-1
This enclosure is damaged after a tornado, but it's a nice big picture to give an idea of what is contained at the rear. From top to bottom:
5 Fans
2 Gigabit switches (Bays 1 and 2)
2 Fibre Channel switches (Bays 3 and 4)
2 Gigabit switches (Bays 5 and 6)
2 Gigabit switches (Bays 7 and 8)
2 Onboard Administrators (the thing that manages the enclosure, one active and one redundant)
5 Fans
6 Power Supply connectors (the actual power supplies are at the front).
So...
The onboard card of each blade maps to Bays 1 and 2, and is always network, but can be e.g. GigE or 10G.
Mezzanine slot 1 has 2 ports and maps to Bays 3 and 4, and can be anything you want, but the convention is usually that this is reserved for Fibre Channel.
Mezzanine slot 2 has up to 4 ports, and maps to Bays 5 - 8, and can be anything you want, but again would generally be network (could be GigE, 10G, Infiniband, etc).
I hope that explains things a bit better.
Some of the benefits of blades are the simplicity of cabling (very few cables coming out the back of the cabinet, compared to the many dozens you would have for the same number of rack-mount servers), reduced power and cooling (because it is shared across numerous blades, it is a lot more efficient), ease of management, ease of provisioning (once an enclosure is racked and cabled, it is dead easy to slide a blade into an empty slot).
