2 years ago, I was in a similar situation as you. I bought a deep blue suit and another medium/dark tan suit thinking they would be the best for interviewing.
Now, those are the two suits of my 12-15 that never get worn.
Now, I'm a financial advisor so my business is obviously much different thank yours, but over 50% of my day is devoted to talking to people I've never met before and trying to make a good impression with them. In a sense, I have 5-10 job interviews a week.
I am not going to suggest for a second that I have an advanced grasp of business formal attire, but I'd like to think I have a moderate amount of knowledge.
When I wear my black suits I always wear a patterned shirt and a bright colored tie. It seems to give off a "more approachable" look.
I'll also sometimes wear either of them with a light colored shirt and no tie and that also seems to go over very well (think of Christian from Nip Tuck for lack of a better example).
My goal in mind is that I always want to have a focal point for how I look. Sometimes it's the suit, othertimes it's the shirt or tie. If you have too much going on, you'll give someone a seizure. Too little and you'll look drab.
The biggest thing I do, is find a look I like and try and immitate it. I've seen a few guys wearing an all black suit with light brown Wingtips and that look is hard to pull off but looks pretty good(and different) IMO and the brown helps to soften up an all black suit.
To me, it's about looking as professional and approachable as possible. All but two of my suits I wear regularly are dark Black/Blue/Grey with the other two being light light grey colors.
To simplify, these are the looks I like:
Solid color suit, white or light colored shirt, bright colored tie-Focal point is the tie
Solid color suit, patterned or bright colored shirt, no tie-Focal point is the shirt
Solid color suit, patterned shirt, dark contrasting tie, brown shoes/belt- Focal point is the shoes
Light colored or pinstriped suit, white or light colored shirt with a dark tie- Focal point is the suit