A good pair of blue trousers is the one you reach for when you want to look like you made an effort without actually making one. They sit between jeans and proper tailoring, smart enough for most places that matter, relaxed enough that you can still eat a full lunch in them.
The difference between a pair that lasts and one you give away after a season usually comes down to the cloth and what's holding it together. A denser cotton twill or a wool blend with a bit of structure will keep its shape across the knee and seat, where cheaper cotton goes baggy by lunchtime. Check the waistband is properly interfaced rather than just folded fabric stitched twice, and that the zip is metal rather than the plasticky sort that starts catching after a few wears. Stitching along the inseam should be tight and even, not the wide, loose kind that pulls apart the first time you sit down hard. Lining matters too, a fully lined pair in a mid or navy blue will hang better and won't go shiny at the thigh the way unlined ones can.
Blue trousers pair with almost anything you already own, which is half the point of buying them. A cream or white top keeps things clean and a bit coastal, a fine knit in grey or oatmeal makes them feel like proper cold weather dressing, and a plain white shirt with the sleeves pushed up is the combination I go back to more than any other. They look equally sensible with loafers or trainers, which means they're not fussy about which bag or coat turns up with them.
They suit the kind of occasions that aren't quite smart and aren't quite casual either, work that doesn't need a suit, lunch with people you want to look pulled together for, a meeting you can't quite read the dress code of. Lighter blue cotton versions earn their place in spring and summer, while a darker, heavier navy or indigo in wool or a thicker twill carries you through autumn and winter without feeling out of season.
If you've none in your wardrobe yet, start with a straight leg in a mid to dark blue, somewhere between navy and denim. It avoids looking too casual, hides a multitude of sins through the stomach and hip, and works with the widest range of shoes and tops, so you're not building an outfit around it, just adding to one you already had in mind.
Wash them inside out on a cool cycle to keep the colour from fading patchy at the knees and seat.