In the hot and drier summer months, hydrangea leaves loose moisture faster than they absorb it thru the roots. If the soil is not kept evenly moist at all times, the leaves will eventually turn brown from the edges inwards. "thank" the sun, wind and temps for that. When potted, the shrub may need to be watered more often than it is watered now. To minimize the drying effect of the black plastic pot due to heat from the sun & wind in this location or due to not enough water, make sure the shrub in is morning sun only locations, make sure the location is not windy and test the potting soil several times a day. Consider the effect some walls and surfaces have to "throw" indirect sun/heat towards the locations where the pot resides (if there are any nearby). The plastic pot itself absorbs a lot of heat and roots in contact or near the pot may be getting hot; I placed some black plastic potted hydrangeas in mostly bright shade instead of morning sun to prevent problems and reduce watering needs last year.

The leaf damage does not appear to be a big/threatening issue yet, especially considering that the shrubs will soon go dormant and loose the leaves "soon". Just try to keep the potting mix as evenly moist as possible (less times where the top of the pot dries out then it gets wet when you water, then it dries out, etc).