What do you see in an inkspot? Rorschach generation'
Python

Not quite the best or smoothest, but here's a python solution:

from PIL import Image
import random
import sys

imgsize = (int(sys.argv[1]), int(sys.argv[2]))
color = (0, 0, 0)
img = Image.new("RGB", imgsize, "white")

for j in range(0,int(sys.argv[3])):
    start = (random.randrange(0, imgsize[0]/2), random.randrange(0, imgsize[1]))
    point = start
    img.putpixel(point, color)

    blotsize = random.randrange(0, int(sys.argv[4]))
    for i in range(blotsize):
        directions = [(point[0], point[1]+1), (point[0], point[1]-1), (point[0]+1, point[1]), (point[0]-1, point[1])]
        toremove = []
        for direction in directions:
            if direction[0]&gt;=(imgsize[0]/2) or direction[1]&gt;=imgsize[1] or direction[0]&lt;0 or direction[1]&lt;0:
                toremove.append(direction)
        for d in toremove:
            directions.remove(d)
        point = random.choice(directions)
        img.putpixel(point, color)

cropped = img.crop((0, 0, imgsize[0]/2, imgsize[1]))
img = img.transpose(Image.FLIP_LEFT_RIGHT)
img.paste(cropped, (0, 0, imgsize[0]/2, imgsize[1]))

img.save("blot.png")


It just makes a "wandering path" for a blot, and makes several of those.

An example usage:

py inkblot.py width height blots blotsize
py inkblot.py 512 512 20 10000


And some example images: