Blepharomastix griseicosta is a species of moth in the family Crambidae. It was described by George Hampson in 1918. It is found in Peru.
The wingspan is 26âÂÂ28 mm. The forewings are silvery white with a grey-brown costal area and a faint brown antemedial line, as well as a curved dark brown discoidal striga. The postmedial line is grey brown with a blackish bar at the costa and there is a grey-brown terminal line. The hindwings are silvery white with a pale grey-brown postmedial line and a grey-brown terminal line, except towards the tornus.