 According to the producers, the Klingon blood was purple to avoid an  "R" rating. Also, the use of purple blood was to serve as a visual  symbol both metaphorical (showing the vast differences between  Klingon and Human values and ideals) and literal (showing the  differences between our anatomies; slamming home why McCoy could  never have saved the Chancellors life). Klingon blood has always  been red in the television series.