About the only way to do it is to export the video as a filmstip -- individual image files -- and touching it up one frame at a time, the way Hollywood does (at substantial cost and with much more advanced tools). Considering that there are 30 frames of video for every second of video, you can see that it's really not a feasible process.
For a bruise, Steve's Rotoscoping method is probably the best, though, as he points out, does require some work - the amount of work, depends on the Duration that the bruise is on-screen.
Another method, Track Matte Keying can be useful, but is probably better, if one just wants to soffen skin, or similar, and also entails some work, depending on how much motion is involved.
With Track Matte Keying, one would Duplicate the Clip, place that onto a Video Track above the original, then add an Effect, like Blur (right there, you can see how this would only "soften" the look of the bruise - not as good as Photoshop's Healing Brush, or Clone Stamp. That will create a Blur over the entire Duplicate, but we will address that next. In Titler, create a Shape (about the size and shape of the bruise), and place that Title w/ the Shape, on a higher Video Track, above the original and the Blurred Duplicate. Add a Blur to that Title, with the Shape, to soften its edges. Add the Track Matte Keying Effect to the Blurred Duplicate, and link that to the Title on the next higher Video Track. This Title - the Track Matte - will restrict what we see from the Blurred Duplicate, to just the area of the Shape. Now comes the fun part. As the subject moves, we need to move that Title (the one with the Blurred Shape), to match the movement. That is done by Keyframing the Fixed Effect>Motion>Position, and can be tedious. One might also need to Keyframe the Fixed Effect>Motion>Scale, if the subject gets closer to, or farther from the camera, allowing the bruise to, in effect, get larger, or smaller.
In the above, it is obvious that the bruise will only be Blurred a bit, to soften its look. If the subject has a lot of movement, then one does have a bunch of Keyframing work on their hands. That technique is much easier, if the subject is basically a "talking head," with little movement, and all we want to do is soften their skin. In that case, we would probably create a custom Track Matte in Photoshop, and only have it affect broader areas of skin, like cheeks, chin and forehead, leaving the eyes, and lips, masked out, so that they do not look Blurred/softened - just the rest of the skin on the face.
Now, if one has Adobe After Effects, it has a Clone Stamp, like in Photoshop, and can be very useful, but as AE is an expensive program, with a steep learning curve, it's not a cheap, or easy "fix."