File hosting is really cheap nowadays and it gets constantly
cheaper. Also if you have a big pipe at home, you don't have
to bother anyway. I really doubt that single blocks can be shared
between a lot of files. Maybe a few KiBs of generic padding or header
data, yes, but not much more. Video and audio data is highly compressed
nowadays and other data that is compressible is usually provided
as ZIP, RAR, BZ2, GZ etc. Even if files have large chunks in common,
it is rather unlikely that these are on the same boundaries. For example
if one file has a slightly larger header (a longer comment for example),
the complete following data is shifted down a couple of bytes.
I believe MP3 files are a rare and maybe the only example in which
case it can be useful to separate the metadata (ID3 tag) from the
actual audio data as people tend to modify the tags whereas the rest
of the data is identical. However, audio files are pretty small by
todays standards and you don't need a lot of sources to download them
really fast. For all other contents (videos, archive, text documents, pictures)
it much less likely that there are a lot of almost identical versions.

Just consider how many possible versions of block of 512 bytes (a typical disk sector size)
exist: 2^(512*8). This is a huge number. This number is magnitudes larger than the number of atoms in the
universe:

2^4096 is this:
10443888814131525066917527107166243825799642490473 83780384233483283953907971557456848826811934997558 34089010671443926283798757343818579360726323608785 13652779459569765437099983403615901343837183144280 70011855946226376318839397712745672334684344586617 49680790870580370407128404874011860911446797778359 80290066869389768817877859469056301902609405995794 53432823469303026696443059025015972399867714215541 69383555988529148631823791443449673408781187263949 64751001890413490084170616750936683338505510329720 88269550769983616369411933015213796825837188091833 65675122131849284636812555022599830041234478486259 56744921946170238065059132456108257318353800876086 22102834270197698202313169017678006675195485079921 63641937028537512478401490715913545998279051339961 15517942711068311340905842728842797915548497829543 23534517065223269061394905987693002122963395687782 87894844061600741294567491982305057164237715481632 13806310459029161369267083428564407304478999719017 81465763473223850267253059899795996090799469201774 62481771844986745565925017832907047311943316555080 75682218465717463732968849128195203174570024409266 16910874148385078411929804522981857338977648103126 08590300130241346718972667321649151113160292078173 8033436090243804708340403154190336

Estimated number of atoms in the universe:
10^81 is this:
10000000000000000000000000000000000000000000000000 00000000000000000000000000000000

It's a small world, isn't it?