Visual block insert/append doesn't work

Make sure you exit visual block with ESC, not Ctrl C.

Also, Vim does not live update in visual block mode, you have to leave visual block mode to show the changes on other lines.

You will only see:

#Here
is 
text

After you enter visual block and insert a #, but once you leave visual block mode pressing ESC it should look like:

#Here
#is 
#text

Ctrl-V-> select the block -> press I -> type #

then press ESC

more detail:

:h v_b_I

you can see:

Visual-block Insert                     *v_b_I* 

With a blockwise selection, I{string}<ESC> will insert {string} at the start 
of block on every line of the block, provided that the line extends into the block. 

For me the problem was that I was using linewise visual mode (i.e. enter visual mode by pressing V), rather than blockwise visual mode (i.e. enter visual mode by using Ctrl-v). Also note that you have to exit visual mode by using <Esc> (or equivalently by using Ctrl-[ on English keyboards) rather than by using Ctrl-c.