On Red Radishes (normally eaten alone)
Brocho before eating this: Borei Pri Ha'adama
Brocho after eating the proper shiur of this: Borei Nefashos
On Black Radishes (normally not eaten alone)
Brocho before eating this: Shehakol Niyeh Bedvoro
Brocho after eating the proper shiur of this: Borei Nefashos
Source: The Laws of B'rachos (pages 293, 294, 378)