Paste an ordinary poster over an old one and you get both of them. The previous artwork ghosts through, the paste darkens patches unevenly, and the piece you designed is not the piece anybody sees.
What blueback does
Blueback paper carries a dyed blue layer on the reverse. Blue is the efficient choice because it absorbs the wavelengths that would otherwise bounce back through the sheet. The result is a printed face that stays as designed regardless of what is underneath it — for a small premium over plain poster paper.
When you need blockout instead
Blockout stock puts an opaque layer between two printed faces. You need it when both sides are printed and visible: hanging banners in an atrium, double-sided pavement signs, anything backlit from behind by daylight.
If nobody will ever see the back, blueback is enough. If they will, blockout is not optional.
The mistake worth avoiding
Specifying blockout for a wall-mounted poster. You pay for weight and opacity you cannot see, and the heavier sheet is harder to paste smoothly on a large format.