You can spot a good pair of blue shorts from the other side of a room because they sit, rather than cling or gape at the waistband, and the denim or cotton keeps its shape when the person wearing it moves. A poor pair announces itself the second someone sits down: the fabric creases in the wrong places, the hem curls up at the back, and you get that tell tale gap at the waist that means a belt is doing a job the shorts should be doing themselves. Hold a pair up before you buy and give the fabric a proper scrunch in your hand. If it springs back without a crease map, that is a good sign.
Colour wise, the easiest blues to live with are the ones that sit mid tone, somewhere between a washed chambray and a proper denim blue, because they go with almost everything you already own without you having to think about it. A dark indigo is brilliant for layering under jumpers in the cooler months, a lighter faded blue reads more casual and summery, and anything too bright or acid toned tends to pull focus away from your face and towards your legs, which is rarely the effect anyone wants. For pairing, think of blue shorts the way you would think of blue jeans: a plain white or cream top calms them down, a striped tee keeps things a bit nautical without trying too hard, and a loose shirt worn open over the top is an easy way to dress them up slightly for an evening that starts warm and ends cooler. If you only own one pair, start with a mid wash, a straight or slightly relaxed leg, and a length that sits around mid thigh, because that cut works across more ages and more body shapes than the very short or very baggy versions, and it does not date the way a trend cut does.
Season wise, these earn their place from late spring right through to early autumn, for anything from a walk somewhere with decent coffee at the end of it to a garden gathering where you want to look like you made an effort without actually making one. They are less useful for anything formal, obviously, but for ordinary daytime life they cover a surprising amount of ground. Tomorrow morning I would put mine with a plain grey tee, trainers that have seen a bit of weather, and a denim jacket slung over one arm in case the morning turns out colder than the forecast promised, which it almost always does.