There are multiple ways to do an ink bleed in Motion and I find AE tutorials rather boring, and typically unrelated to the usual easier way to do the same thing in Motion...
To "roughen" an edge of a line, just convert the Outline of your path to Image.
In the Library > Content, search for blur or scroll down to find Blur 08 or 09 (09 is probably better) and click the Apply button. Drag the image to the Source well of the Outline.
If you use Blur 09 (which is a darker grey to begin) you might want to turn on Additive blend.
Adjust the width to about 100 and the spacing to about 25% (adjustable) and open the Stroke pane. You will probably want to keyframe (or apply a behavior) to the Brush Scale, Brush Scale Randomness and Jitter to create a "flow" of "ink". Adding some Brush Angle Randomness is a nice touch. [When using any of the Random parameters, clicking on the Random Seed parameter control will change everything up.]
If you need to create a more opaque mask or matte, you can apply a Color Correction > Threshold filter to the Line or its parent group and move the Threshold value down towards 0.
Before:
(Blur 09 with Additive Blend and Threshold filter applied)
After:
If this method is unsatisfactory, there is at least one more (choice of Emitter or Replicator — pretty much the same result).
PS, you can draw your own ink blob shape and use that in the image well. Also, you can create several different shapes and create a kind of "inline image sequence" to have either the Shape outline, Replicator or Emitter randomly choose the various shapes... (it will look something like: https://www.youtube.com/watch?v=bQsmqBocD7M - however, this video used a Font of splatter patterns arranged in the inline image sequence technique).
I am assuming for the moment that you know how to turn this into a matte/mask for your reveal effect. If not - let me know.
