What is the meaning of the phrase 铁血丹心 ?
Although it looks like an idiom expression (chengyu), I believe it is actually a coined phrase, specifically for the TV adaptation of the Wuxia novel (射鵰英雄傳, 1983 TVB Hongkong) you mentioned. 

Literally, 鐵血丹心 can translate to "Iron blood scarlet heart". There is the connotation of the conflict between love and patriotism and such, but its a bit far fetched IMO.