Flung ink. Big smears, medium droplets and fine specks, all landing at once.
Three layers of noise at wildly different scales, one for each size of mark. The large smears are distorted by a second noise field so the blobs come out irregular rather than round. The droplets and specks are put through a hard contrast squeeze that keeps only the very brightest part of the range, which is what separates them into distinct marks instead of leaving a continuous mottled field.
The three layers are combined by taking whichever is strongest, and then a ridged noise field is subtracted from the total, cutting gaps and channels through the spatter so it does not read as one solid mass.
Finally the whole thing is cut to hard black and white with no soft edges anywhere. Where ink lands, the picture is tinted; everywhere else it passes through untouched.