From It's The Great Pumpkin, Sam Winchester
Blood. It’s a motif used throughout Supernatural to represent many things – family, guilt, contamination, infection, danger and death.
In this scene, Sam rubs blood on Dean’s face, to ‘mask’ them from Samhain. Sam, despite his early refutation in Metamorphosis, has obviously continued working with Ruby and drinking her blood. Later in this episode we, and Dean, will see this, and get a rare glimpse of the Sam that Sam hopes he is becoming. Strong, capable of defeating powerful evil, the man who can save the innocent. The man he hopes Dean will see him as.
It’s a moment full of symbolism. It’s Sam taking charge; Sam taking care of Dean and he’s doing it with blood. Blood is what links Sam and Dean through family and blood – demon blood - is what is tearing them apart. Here, for a moment, blood saves them both. Sharing blood, they are both the same.
It’s an intimate moment, as Sam daubs the blood on Dean’s face. Dean does something that becomes increasingly rare over the next two seasons – he trusts Sam without question.
And while the boys wear masks here to hide from Samhain, they’re are also wearing masks that hide them from each other.
We all wear masks of course, different masks with different people. With those closest to us sometimes we take them off sometimes – less often really than we’d like to think. Sam and Dean have always worn masks with each other and over 5 years we’ve seen the time when the masks are taken off, or when they slip off, but at this point in their lives I think the real issue is that each is wearing a mask means when they look in the mirror (another recurring image in S4) they can no longer see who they are. They’ve both lost touch not only with his brother, but with himself.