I'm not the definitive source for information on this subject, but I can think of several reasons I wouldn't want to do this:
1) Blowdown is used to get rid of the sediment in the boiler if I'm not mistaken. This sediment will leave the boiler much easier if it is flowing downhill. Also, with an intermittent blowoff (uphill), some sediment may not make it to the blowdown tank and will either trickle back down the line with the rest of the liquid when the valve is closed or settle somewhere in the middle of the pipe (either way, not good).
2) As I mentioned above for an intermittent blowdown, your pipe from the boiler to the blowdown tank will not empty on its own. The fluid in the pipe will drain back to the blowoff valve. I can't tell you if this is good or bad, but I would think you wouldn't want this situation.
Sorry I can't answer your question. I'm sure your proposed design could work if the proper measures were taken to avoid problems (I'm sure it's been done before). Hopefully someone else can give you some better information!
Best of luck!
Jproj