The optical method of caustics for a blunt crack (or keyhole crack) is analyzed. The stress field near the keyhole crack is expressed by the Williams-Irwin series expansion including higher order singular terms than the usual r-1/2 singular term. The coefficients of the higher order singular terms are determined by the boundary collocation method for K1 field dominance at far field. The numerical and experimental caustics show good agreement in shape and size. This analysis concludes that the effect of bluntness is significant only within the distance four times the notch root radius from the notch root. If the initial curve of a caustic lies within the distance, the stress intensity factor is highly overestimated by the caustic formula of a sharp crack.