The Meaning of Life, the Universe, and Everything.
Does anyone know a good way to create a skyblock map from scratch? I found some (old) guides suggesting to create a superflat void world, but the problem with that is the biome of the whole map is set to the void, while the original skyblock still had different biomes for different (empty) areas that could be used to your advantage (mob farms etc...).
Download NBTExplorer - this program allows you to open .dat and .mca files. It's really straightforward to use.
First, you need to know exactly what blocks need their biome changed, and more importantly, what chunks those blocks are in.
Next, open NBTExplorer and go to saves/YourWorld/region. This "region" folder stores all the information about overworld chunks, including biome data. Next, you'll need to know what chunks you want to change - you can find this out by hitting F3 and looking at "Chunk" when you're in those blocks - specifically the x and z values (you can ignore the y values).
Take that chunk, divide both coordinates by 32, and round down (so x=32 becomes 1, x=31 becomes 0, and x=-1 becomes -1) - this is the .mca folder that you want to open. You will see a list of folders labeled "Chunk[x,z]" - ignore this, and instead look at the smaller print to the right that says "in world at [x,z]" - pick the folder whose smaller print matches the chunk you want to change. If you can't find it, you're probably in the wrong .mca folder.
If you have the chunk that you want to modify, open that folder. Then open the "Level" folder, and you should see in there a file marked "Biomes". Open that file and click "text view" - this will give you a table of 16x16 numbers, which correspond to the biome ID of each block in the chunk (chunks are 16x16 blocks). I'm pretty sure that left-to-right is increasing x and top-to-bottom is increasing z, but I'm not sure - unless you want to make the whole chunk the same, you might need to test which numbers correspond to which blocks. Anyway, modify these numbers to whatever Biome IDs you want.
Remember to click save!
Then open the world and see if it worked!
If you want large areas to be changed to a different biome, you might want a different program, but it seems from what you're saying that the biomes might be fairly small. Otherwise, you can always copy/paste the "Biomes" file from one chunk to another to save some time.