I think the recommendation there is to place all block textures in one sf::Texture, set it once and then just change the texture rect instead.
That said, what is
block?
Looks like you have a
lot of those blocks being drawn separately. You may want to consider using a form of tile map (which draws everything at once). There is
an example of a simple tile map in the vertex array tutorial. However, make sure you read
the entire tutorial This, though, still draws each block as a separate 'quad' so the gaps (or 'borders' as you called them) between tiles can still appear. This has been discussed a lot in these forums. The most basic way is to use strict values. That is, round all positions, lock rotations to 90° steps, use scales of multiples of 2 and the same applies for the view (position, rotation, size). A more robust and flexible (but slightly more complicated) way would to be to render to a render texture first (with strict values as above) and then draw the render texture to window with more free values (offset positions and scales, for example, can be non-integer). This removes the gaps.
Just as a footnote,
Selba Ward's Tilemap does all this automatically