In YDSE, the source is placed symmetrical to the slits. If a transparent slab is placed in front of the upper slit, then (slab can absorb a fraction of light)

Due to absorption of light by the slab, intensity of two sources would be different and hence intensity at central maxima changes. If no absorption takes place, then it will not change. Intensity of dark fringes will not be absolutely zero, if some absorption is there. The central maxima shifts towards the slit which is covered.