Fungal disease! You'll probably have to remove those affected leaves, otherwise it will spread bottom-up to the top leaves and by late August you might have none. This is common for arborescens but doesn't seem to happen every year. (The disease will be eradicated by winter.)

If you can detect, with magnifying glasses, signs of fungi, it may be anthracnose (a fungal infection) but if you are located in northernmost states, it could be damage from late frosts.

H. arborescens can show leaves with black lesions due to late frosts; if caused by late frosts, replacement (new) leaves will be fine and not develop the black lesions again. Sounds weird to be talking about late frosts though... where are you located???

See the link below for info on fungal diseases:

http://www.aces.edu/pubs/docs/A/ANR-1212/index2.tmpl

If the plant got dry enough and gets too much sun, it could be moisture problems since Annabelle tends to be one that likes afternoon shade. I haven't seen leaves like that with my Annabelle, so just trying to list all possibilities.

This fall I would do a really good cleanup of old leaves and then put down fresh mulch just in case it is a fungal problem, since often fungal spores overwinter in plant debris.

luis_pr I am located in Ottawa Canada (Zone 5a). I don't think it is a late frost. Our late frost was in the beginning of May. I did actually fertilize my hydrangea 2 weeks a go with ultra bloom miracle grow. I have removed many affected leaves waiting to be replaced by a new ones.

Just a theory, but could the combination of hot, dry weather and the salts in the fertilizer have caused this issue? I don't use fertilizers, so I don't know.