The blocks are a holdover from our early UM200 systems before we put in microprocessors. The frequency switches directly controlled a parallel input PLL (frequency chip). Since we had 256 possible switch settings using two 16 position switches and we were making 100kHz steps, we defined a block as 25.6 Mhz in size. Block 0 would have gone from 0Hz to 25.5 Mhz, Block 1 from 25.6 to 51.2 MHz and so forth. So the beginning frequency in any of these blocks would have been the block number times 25.6 MHz and the last frequency in that block would 25.5 MHz above that.
So, in a sense our blocks were pretty arbitrary and have little to do with the world outside Lectrosonics. As far as the outside world, you can call the sales department at Lectrosonics (800 821-1121), give them your location(s), what other wireless gear you have and they will make a good recommendation. If they mess up and you are using our recommended gear, we then have the responsibility to get you out of trouble. Our dealers are pretty good at frequency selection since they live in the neighborhood.
The other valuable resource is the RAMPS group since you can probably find someone that is operating in your area and can tell you what works. (See info on RAMPS in FAQ What is RAMPS?)
I assumed that you were speaking about our gear since you mentioned blocks. If you are interested in other brand equipment, call their offices and get advice for your area. The other manufacturers are generally quite helpful and if they aren't then try some one else.