What does 狗血 mean as an adjective?
I think (洒)狗血 is from Taiwan originally. 狗血 could be short for 洒狗血.  

洒狗血 could be interpreted in Chinese as 瞎搞 or 乱搞. In English, it could be something illogical, trying to be funny in a silly way, utter nonsense and etc. 

So, 剧情狗血 could be the plot of the drama is ridiculous, utter nonsense, stupid and something like that. 

撒狗粮 means a couple try to show how they love each other in front of a single person. 

单身狗 means one who is single. So, you could say 在单身狗面前撒狗粮.

远光狗 refers to those always turning on the high headlight beams while driving, which dazzles vehicles from the opposite direction.  

I don't usually hear 远光狗, 单身狗, 撒狗粮. I hear 剧情狗血 and 洒狗血 occasionally.  

远光狗 is considered to be an insult, while 单身狗 and 撒狗粮 take some humorous effect I think.