I think this is highly counter-intuitive, as it is not explicitly called out in game, and the graphics suggest that both sides will be blocked. Similarly the graphics suggest that inserters will be blocked, when they in fact are not.
Since it seems to be the developers intent to not add extra belt items to handle different cases, I think they should redraw these sprites such that only the one side of the belt is visually blocked, making it clearer that items can in fact be loaded from the other side.
viewtopic.php?f=6&t=23791#p149506
sporefreak wrote:May not be as convenient as an item to block half of a conveyor but this works well for some situations.
Especially that one you have there.
The underground conveyors are "Blocked" on the back half. (Inserters can still place and take from them without any problems.