  By coupling a probe transition to a Rydberg state using electro-magnetically
induced transparency (EIT) we map the strong dipole-dipole interactions onto an
optical field. We characterize the resulting cooperative optical non-linearity
as a function of probe strength and density. We show that the effect of dipole
blockade cannot be described using a mean-field but requires an $N$-atom
cooperative model. Good quantitative agreement is obtained for three atoms per
blockade with the $n=60$ Rydberg state. We place an upper-limit on the
dephasing rate of the blockade spheres of $<110$ kHz.
