Can't find mention of Black Caves or Cape Merde (or indeed Merde Creek!) in either of those two guidebooks Feen. Are you sure it's Symi you're going to?
"Maps" of the island are available at kiosks in Yialos. Don't touch them - they're drawn by Greek mapmakers with a very odd sense of humour!
Get Lance Chilton book even if you don't intend walking, it's worth the price just for the accurate map that comes with it.