John is right, the Blast top end is made of leftover Buell tuber parts, not XB parts.
The Blast came out in 2000, well before the XB line which came out as an '03 model. So even though it's lower end is similar to XB's, and has the 3-1/8 stroke of the XB9, the top end is the older tuber stuff. The head is actually the old "Lightning" head, with a 62cc chamber and 10 degree squish shelf and smallish valves (1.715 intake, 1.580 exhaust). The piston is the 15 degree domed "Thunderstorm" piston. They paired the small chamber Lightning head with the domed Thunderstorm piston because shortening the stroke took away a lot of compression, and those were parts they had sitting around that could get the compression back to something workable. It's still only 9.2:1, though.
The factory mismatched 15 degree dome and 10 degree squish shelf creates something of a fuel trap at the o.d., but honestly, from the factory there's usually so much squish clearance that it doesn't matter anyway.
When they came out with the XB9 they did a better job, a pear-shaped 62cc bathtub chamber with flat (zero degree) squish bands on each side, and a bigger rectangular dome on the piston that sticks up into the chamber, giving 10:1. Not the best design, as fuel coming out of the squish band hits the dome and has to turn a corner, but it works reasonably well and it's easy to manufacture. Very easy to cut a 15 or 30 degree shelf into that head and use a different piston, though.
I fiddled with one of these motors when they first came out. They're pretty tough. Here's what I ended up getting out of, at 515cc:
We took it to the salt flats once, but broke it. At the time, there were no single-cylinder ignition modules available that would allow me to raise the rev limit from the factory 6500rpm number. So I used a module off a Big Twin, that had a 5500rpm limit. Since it was only seeing half the pulses, the rev limit effectively was 11,000 rpm.
Well, we got the jetting dialed in, and my wife, who was riding, took off, and the motor was set up to be peaky as hell, and when it came on the pipe, it
really came on the pipe, and spun the tire, and overrev'ed the motor, and dropped a valve. I found the tip of the valve (from the groove up) amidst the debris, it tore it right off.
I put it back together but never did drag it back out to the salt, eventually detuned it for street and sold it.