CGI shadows

i have set up a camera track for a video and a plane for my ground, with it set to shadows only. I have a sun lamp, with blue shadows, but when I render it, the shadows are black. I try turning off the only shadows, and it works perfect with blue shadows that matches my scene. However, once i turn on shadows only, the shadows become black.

I can tell theres something obvious im missing, so sorry if this is a derpy question…:o

can anyone help me out?

EDIT: I am using the original blender render, by the way.

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