  We explore a single-photon approach to Rydberg state excitation and Rydberg
blockade. Using detailed theoretical models, we show the feasibility of direct
excitation, predict the effect of background electric fields, and calculate the
required interatomic distance to observe Rydberg blockade. We then measure and
control the electric field environment to enable coherent control of Rydberg
states. With this coherent control, we demonstrate Rydberg blockade of two
atoms separated by 6.6(3) {\mu}m. When compared with the more common two-photon
excitation method, this single-photon approach is advantageous because it
eliminates channels for decoherence through photon scattering and AC Stark
shifts from the intermediate state while moderately increasing Doppler
sensitivity.
