Source: http://www.phillymag.com/eagles/2012/08/23/twitter-mailbag-eagles-bring-back-the-black-jersey/ (Via Uni Watch Ticker)
They last wore black uniforms with green pants was on January 2, 2005 against Cincinnati.
"...One thing about the '69 - '78 Redskins though...the sleeve stripes on the burgundy jerseys should have the wider yellow stripes as you have on the '70 - '71 images. The '69 and '73 - '78 all have equal width stripes. "
"... I haven't noticed any evidence of two styles of striping like they had on the white jerseys from '74 - '78."