A blind run refers to a let's play of a game which the author hasn't played or finished before. This is used to capture the feeling of exploration one may get when playing certain games for the first time. Its main strengths are the author's honest and spontaneous reactions to game events, while the downside is that by definition there can be no planning or prior knowledge, which can be frustrating and boring to the viewer as the author may struggle to progress and miss game content.
Some let's players will try to feign, explicitly or not, the element of surprise from blind runs by feigning ignorance. While this helps prevent spoilers, it requires a great deal of competence and if done badly may feel quite fake and cheesy.
It is unclear as of yet which types of games are better suited for blind runs. Story-heavy games, for example, might be considered suitable as the author would react progressively to advancements in the plot. On the other hand, it might be said that a player that already went through the game on many occasions may have a deeper grasp of the more subtle narrative aspects. In the end much depends on the style of both author and game.
Scare games naturally lend themselves to blind runs, since they revolve around surprising the player - a reaction that is very hard to fake. It can even be argued that much of PewDiePie's success is due to his exaggeration of this effect.