I understand if a file has 1 byte, it will still take up an entire block on disk (e.g. 4KB). Is the same true for a zero-length file? I am specifically wondering about NTFS but insight on other file systems welcome!
No, in case of NTFS, if file has 1 byte, it doesn't use any block. In general, if file has less than 300 bytes (approximately and in case that file record in MFT has 512 bytes - this value depends on file name length, size of MTF file record, etc.), data are located in MFT (master file table). Only if it doesn't fit in one file record (in MFT), then data are externalized to blocks (usually 4 KB).