In this generator Brownian Noise is used to place blocks in a random pattern where each position has a higher probability to contain a block when the surrounding positions also contain a block. By changing the seed you can get different patterns, with the density you can increase or decrease the number of blocks.

If you want to know how it works you can look here: https://en.wikipedia.org/wiki/Brownian_noise

Edit: Improved the script performance so higher block numbers can be generated in a reasonable time.