Depends on what you mean by "block". Your OS is probably interacting with the SSD in terms of 512-byte logical blocks, but most likely grouping most I/O into 4kB or larger chunks due to the 4kB virtual memory page size. But the underlying NAND flash has a native page size of more like 16kB, and an erase block size in the MB range, and an internal RAID-like striping of data across multiple channels on the SSD controller. Sequential access is still much simpler and faster than random access.