Blocks, or sectors, on the HD are determined by the HD itself and have traditionally always been 512 bytes.
So-called AF (advanced format) drives are an exception as they have actual 4096-byte sectors, but often still use block addressing based on a 512-byte sector size to be compatible with current OSes.
The allocation unit size as reported by Windows is a filesystem feature and is the same as the cluster size. By default, clusters on an NTFS filesystem are 8 sectors each, or 4096 bytes (4k).
In your case, you're using the exFAT filesystem and a cluster size of 256KB. This is unusually big and will lead to a lot of lost disk space if you're storing many small files on the drive, but it does give a slight performance boost with sequential reading/writing of very large files.