What is the meaning of the phrase 铁血丹心 ? This is the title of a famous wuxia-novel, whose full title is 射鵰英雄傳之鐵血丹心, it is also a the name of a Cantonese song. 

I have searched for an explanation of this phrase.
I thought,  maybe it is a chengyu, an expression in classical Chinese, or a quotation from some text, but I did not found any clue for being so. Maybe the author itself invented this phrase. 

I would like to understand the meaning of this  phrase, 
by this I mean its  grammatical structure, cultural significance or any other annotation that  helps to shed some light onto this phrase.
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.