Tensile tests and simple shear tests have been performed on Armco iron sheets before hydrogen cathodic charging, in order to study the effect of loading mode on blistering of plastically prestrained samples. The experimental results from surfometry and image analysis show the decrease of the blisters size with prestrain increase, and quantitative differences between tensile or shear prestrain are exhibited. Phenomenological modeling of blister initiation is proposed, based on void expansion increase under internal pressure, and considering both plastic strain and pores as traps during hydrogen transport. Finite element simulations of material expansion due to hydrogen induced void growth lead to qualitative agreement with experimental tendencies.