roast duck leg with plum sauce

Duck generally has a considerable amount of fat, so there will probably be a good deal of fat in the pan drippings.

That's not going to be a problem if you just blend the plums themselves; they're already full of water (i.e. juice), so they're not going to actually absorb a significant amount of the fat, they just might have a little fat film on the part that was touching the pan, which you can brush or wipe off. I personally wouldn't bother, just toss 'em right in.

Just don't throw the pan drippings in the blender with the plums, because those will be chock-full of fat and you'll end up with a very cloudy and greasy sauce. Instead, very carefully pour off the fat (but not the juices!) from the pan, or better yet, while you're blending the plums, pass the drippings through a fat separator (AKA gravy separator). Also be sure to deglaze the pan if you see any burned-on bits, because those will add a ton of flavour.

Then combine the blended plums and the fond (meat juices and deglazed bits) and reduce it. You can and should reduce it to thicken it - that's how most pan sauces are made.

As for keeping the meat hot - you should be able to reduce the sauce fairly quickly since there isn't much in it that will burn at higher temperatures, but if that's still not quick enough, then I would just cover the meat and pan with foil and keep it in a warm (150° F / 65° C) oven. Leave the lid to your oven open while you start preparing the sauce so that the temperature can come down to "warm" quicker.