There’s no reason why either the adjective or the noun should bear an initial capital letter, unless it forms part of the name of something, such as the organisation you mention.
It's a fair question, I suppose – I mean, I believe you were told what you said you were told. However, I'm having trouble imagining anyone on this side of the Atlantic who would be confused by this use of the word key.
As a matter of fact, sportswriters don't usually write for a sophisticated audience, but the Boston Globe wrote about:
Three key plays that won the Super Bowl for Patriots
and the USA Today published a story called:
Gronkowski key part of Patriots' winning Super Bowl
More recently, a sportswriter analyzed Game 1 of the NBA Finals, saying:
Rebounding is a key in any series in basketball, and that figures to be one of the biggest advantages for the Cavaliers. However, it was Golden State who won the battle on the glass, outrebounding Cleveland, 48-45.
This isn't just a sports thing, either. From a recent New York Times interview with a publishing executive:
Every opportunity is a different challenge, and I will quickly assess what the key issues are...
In short, either your friend had an erroneous perception, or else he was pulling your leg.
Best Answer
An adverb can modify an adjective, verb, or another adverb.
The meaning becomes clear when you take the two words and use them in a sentence.
It's blatantly obvious my dear, that you are smashed. Here, give me that other shoe. Now, let's get you to bed.
In the sentence obvious, as a predicate adjective after the linking verb is describes it [the fact she was intoxicated] as an adjective to mean it was easy for him to see and understand what her condition was. It's modified by the adverb blatantly to show "To what extent was she drunk?" "Drunk enough so that she could hardly stand or sit to take off her shoes, much less make it to the bedroom on her own."