Why are some Unix systems built around 512-byte blocks, whereas others are built around 1024-byte blocks?
This is all because of the history and evolution of Unix systems. When Unix was first deployed, disks were small, and it was important to squeeze as many bytes out of the disk as possible. As a result, the file system was developed with a fundamental block size of 512 bytes (that is, the space allocated for files was always in 512-byte chunks). As disks became bigger, millions of 512-byte blocks began to prove more difficult to manage than their benefit of allowing more effective utilization of the disk. As a result, the block size doubled to 1KB and has remained there to this day. Some Unix systems have stayed with the 512-byte historical block size, whereas others are on the more modern 1KB block size.
Do all device names have meaning?
As much as possible, yes. Sometimes you can't help but end up with a /dev/fd13x4s3, but even then there's probably a logical explanation behind the naming convention.
If there's a flag to du that causes it to report results in 1KB blocks on a system that defaults to 512-byte blocks, why isn't there a flag on 1KB systems to report in 512-byte blocks?
Ah, you expect everything to make sense? Maybe you're in the wrong field after all....