There are very strict rules concerning a gentleman's attire.
If you're pairing a pair of odd trousers with an odd jacket, you must NEVER match the colour! This is a very strict rule, there's absolutely no wiggle room here. If you're wearing a navy blazer, the best colour trousers to pair it with are khakis, cream coloured, ash, stone, light grey, mid grey, dark grey, all the way to charcoal grey. But NEVER navy trousers with a navy jacket, UNLESS it's a full suit. I cannot stress how important this rule is. My personal favourite trouser colours for a navy blazer are stone (a very, very pale shade of cream/light brown) or light grey. It's sharp and classic.
So, just remember the rule - where you decide to wear an odd jacket with odd trousers, you always want as much contrast as possible between the jacket and the trouser. For instance, I love pairing a tan jacket with stone trousers, or a navy jacket with grey trousers, or a charcoal jacket with light grey trousers.
When it comes to suits...it's an entirely different matter. Just remember that all the components of a suit (i.e. trouser, jacket and waistcoat) are made from the same length of cloth so they are completely identical...that's the basic definition of a suit. It's a whole world of options and possibilities. Anything from navy suits, light grey suits, charcoal suits, pinstripes, chalk stripes, tan suits, brown suits, different weights and types of cloth (British new wool, cashmere blend, silk blend, cheap synthetic fabrics, synthetic/wool blends, pure linens, linen/polyester blends)...the possibilities are endless. The rules governing the appropriate way to put together a suit outfit are completely different from the rules related to wearing blazers and odd jackets. I could talk about it extensively but it would only serve to bore you. But it seems like we're not talking about suits in this thread, rather blazers and odd trousers, so lets stick with that.
So, let me just sum up: the navy/blue blazer should be a staple item in every man's wardrobe, for it is a timeless classic. It goes perfectly when paired with light grey, mid grey, cream, stone, ash, or charcoal coloured trousers, or even khakis and chinos if you're looking to go down the casual route. You have a lot of options here, and the colour you pick is entirely dependent upon your taste (there's no right or wrong). But avoid navy or black trousers at all costs! Unless you're pairing them with a light coloured jacket (e.g. tan or light grey), in which case you'd have yourself a sharp, classic outfit.