What is "block-compressed" file in samtools?
A block compression usually refers to compressing your file into a series of small blocks (with a tool like bgzip). This allows indexing in that the index can record which record lives in which block so that the whole file does not need to be decompressed and therefore can be accessed randomly. This isn't true of files compressed as a single block using gzip. 

As far as I was aware samtools only works with uncompressed FASTA files.