BlueGrid 2 is a small application that can be used to keep page elements lined up on the grid, both vertically and horizontally in Blueprint.
Blueprint comes with a background grid image to use for this, but it only works as a background, and it is not that useful when you are working with colors and final tweaks.
The nice thing is that BlueGrid 2 is semi-transparent, draggable and resizable. BlueGrid 2 floats to the front, so you can even edit behind it while it is running.