vaccum flow direction question

[patrickx5]

Part No. 6 in below diagram is a one-way flow do-hickey; came off in my hands while doing valve cover gasket. I don't want to learn the full functionality of the vacuum system, I just want to know what direction the air flow is so that I can place it correctly (had to replace the vacuum lines; they fell apart). Would appreciate if somebody could inform me! One side of the part is black & what side is white; it works presumably since I can blow through it in one direction but not the other.