An opaque adhesive can be used to secure the convex surface of a lens blank to a lens blocking piece. This opaque adhesive can include a resin that is curable under ultraviolet light and contains many dispersed pigment particles. The pigment particles may absorb light in the wavelength range of about 300 nm to about 800 nm such that the adhesive appears substantially opaque. A lens-on-block (LOB) system that uses the opaque adhesive to adhere a lens blank to a lens blocking piece facilitates on-block inspection of surface defects by increasing the contrast of light reflected from the surface being inspected. Put differently, the defects in the lens blank's surfaces stand out more when viewed in front of the opaque adhesive The inspection results can be used to carry out rework loops that eliminate or correct the detected defects, providing a streamlined method for on-block manufacturing of ophthalmic lenses.