First I'll determine the blending proportions. I'll do this by pulling off a pint or so of whatever beers I'm blending. I'll set up small sample glasses and very carefully blend different percentages. Typically a lambic will be 70% 1-year old with 30% six month old. Anyway, whatever the final proportions, your looking for the best balance and greatest complexity.
I keg all of my beers so the process is really the same as normal kegging. After cleaning the keg is purged with CO2, which is heavier than air so the cap can be removed without risk of losing CO2. As the beer is racked into the keg, the liquid forced the CO2 out of the opening at the top. I have a cvpvc measuring stick with markings on the side for the various kettles, mash tuns, liquor tanks, kegs that I use. The markings are calibrated at 1gal increments so I can determine the exact volumes of the various vessles. I will apply the final proportion to the blend, keg the beer, and purge the head space with CO2. After carbonation, I'll use a beer gun to bottle.
I've used a similar method when bottle conditioning as well. I'll take my stainless steel boil kettle and follow the same procedure noted above. Once I have the racked beer in the kettle I'll mix in the boiled, cooled, DME and additional yeast being carefull not to splash. I've never really had a problem with oxidation. Good Brewing - Cheers!