We bought this Hydrangea in May this year in a gardening store. It stayed outside on the balcony all the time. After a while the flowers started to lose blue color, so we bought a fertilizer with aluminium to keep the flowers up a bit. Eventually, the flower branches bent down and we cut the off leaving only the leaves. The plant doesn't seem to grow any new leaves since then. It looks like it slowly withers. We water it regularly, in fact it has now these small water dispensers inserted in the ground. The water comes from a tap but is kept for a few days before it is applied. Our tap water is very hard unfortunately, could it be the reason for leaves becoming dry? We also use a special soil for this type of Hydrangea bought in the same store.
I live in an area with very hard water and soil of alkaline clay, and my hydrangeas grows excellent in shade.
Most of the plants was bought with blue flowers exactly like yours, but next year they get pink flowers because of the alkaline ground. If you prefere blue flowers an easy solution is just to collect rain for watering. I do not think it make sense to store the tap water a fews days to make it softer. If you are out of rainwater then try using boiled (but cool) water e.g. the water you did not use for tea. Observe, that after some weeks the flowers often turn greenish. This is a natural process, and you can cut of older flowers. New flowers will have strong colours.
A sunny and hot balcony is certainly not optimal for most hydrangeas. However, the damaged leaf on the photo is very common when you have hydrangeas in undrained pots especially indoor, and first they get too much water and are soaking for days, and then they get to less often because of damaged roots and often in combination with too much sunlight. Too much fertilizer in combination with too much sunlight can cause similar damages.
Hydrangeas should not dry out and need frequent watering. If you have an undrained pot, then remember to empty it 20 minutes after watering. I would be very careful with water dispensers.
I you use a smaller amount of organic fertilizer (no added Ca) in the spring and perhaps one time in the summer, if the leaf is not dark green.
Insufficient water is one problem, but I can see tiny white blobs, maybe insects or eggs, on the underside of the top leaf in the second photograph - I can't tell what they are, but check with a magnifying glass and check the underside of all the leaves. If there is some insect activity going on, treat for that, but increase your watering anyway, especially now the pot has drainage holes. It's better to water thoroughly with tap water regularly than to wait to use lime free water. It may also need a larger pot...does better in shadier situations than in full sun.