Want to build a structure (wall, road, whatever) from an assortment of different blocks, with the result looking like the blocks have been placed randomly? Then, this mod is for you.
After you turn it on, every time you place a block, your hotbar will select a different (or possibly the same) slot randomly, so your next placement will be that different block.
In easy mode, you just press the 'r' key (can be changed in options). From now on, whenever you place a block, your hotbar will switch to a different block. Press 'r' again to turn it off.
For more fine control, you can use a command. For example, you want to build a wall from cobblestone, stone, and bricks, and manually insert a lantern from time to time. Place a stack of those three blocks into each of three adjacent hotbar slots. Then tell the mod which slots you're using - the command for that is /rblock <slotno> <slotno> with slot numbers counting from 1 to 9. So if you're using the last 3 slots, the command is /rblock 7 9.
Now just build your wall; each time you place a block, the hotbar gets a random switch betwen one of those 3. If you placed the lantern in some other slot, the mod will neither switch to nor away from that slot, so you still have manual control over where you place that.
To turn the thing off, press r again, or use /rblock off if you prefer a command or haven't bound the key.
You can use different ratios of blocks in your hotbar to get different ratios in your build. For example, 4 stacks of cobble, 2 stacks of stone, and one stack of bricks in slots 3 to 9 - build a wall of mostly cobblestone with some stone and a few bricks interspersed.
The randomizer checks the number of items in each slot as well; this creates better distributions when the number of items is small. For example, if you have 50 cobble in one slot and 5 stone in another, the stone will have a 1:10 chance to get selected, so it should last approximately as long as the cobble, and every 10th wall block will be a stone block (not every second as you could assume with just 2 slots in use).