As far as I know, you’re right, CC Mr. Mercury does not easily lend itself to creating a transition since it doesn’t have a “master” knob for switching from a blobby image to a flat, unaffected one.
You have several options…
One simple method is to tweak the parameters to get an appropraite transition. Try something like this:
X & Y radius: 90 (depending on the size of your comp this may need to be more, or less)
Producer: (in the middle, which is the default setting)
Direction: (whatever, it’s irrelevant here)
Velocity: 0
Birth rate: 30 (depending on how fast you need the transition to be, you may need to play with that)
Longevity: 4sec (long enough to last the whole transition)
Gravity: 0
Resistance: 0
Extra: 1
Animation: Explosive (doesn’t make much difference with these settings)
Blob Influence: 100%
Influence Map: Blob In
Blob Birth Size: 0.01
Blob Death Size: 2
When this is applied to a layer, it should transition from fully transparent to fully opaque and undistorted, with some wobbly blobby action in the middle.
Another option would be to apply Mercury to a solid white layer. At the end of the animation you’ll want the layer to be its original self – solid white – and if Mercury doesn’t give you that just fade in another white layer, untreated. Then use this white animation as a track matte, and possibly a distortion map, to the layer you need to fade in. This may be too sketchy for a new user so let me know if you need more details.
AA