I've bought a little bit of each, I don't know the official answer but I will share my experiences.
Blanchard Ground: This is surfaced on two sides to be flat and evenly thick. The grind lines run perpendicular to the length of the steel in a circular pattern. This stuff is more than flat enough to do anything you want with knives. I've made folders with it, so fixed blades will be no problem at all. The issue of blanchard ground is the direction of the grind lines, if it was done with a coarse wheel you have deep scratches across the peice before you ever start. It could be a pain in the ass finishing the flats of the your blade. The majority of the mill scale will be gone though.
Precision Ground: From my experience this gets you a peice of hot rolled steel that has had the high spots taken off with a surface grinder. It will not be any flatter than blanchard ground. This is not completely flat. Say you want .125" thick steel, they take a peice that was supposed to be rolled out at .125" thick and surface grind it to take off any high spots. For the most part it will all be .125" thick, but some places might be a couple thousandths under. This will also be more than flat enough for the knives your wanting to make, it is mainly recommended for folder making. One thing in its favor is that the majority of the mill scale will be gone, and the grind lines will run parallel to the steel. This will cut down on finishing time.
Flat Ground: The stuff I have bought has actually been labeled flat ground precision or something like that. It has all been O1 I bought at a local machine tool supply company. This is an oversized peice of steel that has been surface ground down to the desired thickness. There will be absolutely no mill scale and high and low spots will be limited to whatever the tolerances of the machine are. Its much flatter than any steel I've bought that was just called precision ground. It also costs a lot! I would not recommend it for fixed blades for the simple fact that you don't have the need for those tolerances so there is no need to pay for it. Save this stuff for folders.
Hot Rolled: This is straight out of the mill, undressed. Its not really that flat if your checking it with a micrometer or caliper, but is plenty good enough for fixed blades. I've even made folders with it without too mugh difficulty although I would not recommend trying that and wouldn't do it again myself. It will be covered in scale which can be difficult to remove depending on the type of steel (440C isn't too bad from my experience) It should be completely annealed, as should all the steel I listed above. This would be what I recommend for making fixed blades with provided you have access to power tools. If you are filing it and doing the finish work completely by hand, I would recommend you go with the precision ground in order to avoid battling the mill scale with a sanding block
I have not worked with cold rolled but the impression I have gotten is like Roger said, its not as easy to machine as hot rolled. It probably doesn't have the nasty mill scale though.
Hope that helps, someone please correct me if I'm wrong.
