Blue trousers are the pair that solve the problem black trousers create, which is that everything above the waist has to work twice as hard not to look like a uniform. A good blue, whether that's navy, denim toned or something closer to cobalt, gives you the same ease of pairing as black without the slight funeral quality black trousers can bring to a Tuesday. They're the bottom half you put on when you want to look like you dressed on purpose but didn't spend long doing it.
The difference between a pair that lasts and a pair that bags at the knee by November usually comes down to what's holding the shape, not just what the shape looks like on. A wool or wool blend keeps a crease and recovers after sitting down all day, cotton twill is sturdy but needs a bit of structure in the waistband or it goes soft and shapeless, and anything with a decent amount of weight to it will hang better than something thin and floaty that catches on tights. Check the waistband is faced rather than just folded fabric with elastic jammed in, look at whether the fly is a proper zip with a hook closure rather than just a button fighting gravity, and run a hand down the inner seam, if it's overlocked rather than bound it'll fray eventually, just not on day one so you won't know until it's too late.
Blue trousers are forgiving about what goes on top. A cream or white shirt is the obvious answer and a good one, but a grey marl jumper works just as well and a patterned blouse in warm tones tends to sit nicely against a true navy without the two fighting each other. Tan or brown shoes lift blue in a way black shoes never quite manage, there's a warmth that comes through.
They're suited to the stretch of the year where you need something a shade more considered than jeans but it's still too warm, or too undecided, for a coat that does all the talking. Office days, a lunch that matters, the kind of weekend where you might end up somewhere nicer than planned. Deep winter wants wool, the shoulder seasons are where cotton and linen blends earn their place.
If you're starting from nothing, go for a straight or slightly tapered leg in a mid to dark navy, in wool or a wool blend, with a proper waistband. It's the one that won't date, won't crease into a mess on a train, and goes with roughly everything you already own, which is the whole point of buying one pair before you buy five.
Wash them less than you think you need to and let a steam or a hang in a steamy bathroom do most of the work between washes.