'In the summer the leaves curl and turn black. '

Sounds like it suffering from the drought.
All clethras prefer to be in a moist acidic soil.

My guess is the need for moisture retentive, acid soil. Bloom might not be as good as it would be in full sun, but then you have to be sure the soil does not dry out. Clethra are common in the shade here, spreading into colonies with almost no bloom and are covered with flowers at the edge of the road, but always in low woods and acid fibrous soil

My plant is in its second spot in high shade of a mature hickory (20 feet?)with fast-draining-not-very-rich soil. I supplement water every several days with a long soak and start the season out with some hollytone. It does withstand periods of dryness now. Flowers well but took maybe three years to establish. It died back to almost nothing the first or second year due to neglect. It bloomed the first year then skipped a year or two. Now it blooms well. It really needed some special attention and not to be smothered by other plants that first two years.
So you have a few choices here.

Give it the third year to establish, keeping it well watered as stated above.
Try another bright shade spot, out of hottest sun of the day, and some distance from competing tree roots. (What kind of tree is it?) While your moving it, examine the roots. How do they look? How are the stems? Are the healthy or do they have some cankering? Clethra is suppose to be disease resistant but I can find a reference to phytophthora root rot at one of the college sites.
Additionally, check for spidermites. You might be able to tell this late in the season if you look very closely at places where the stems branch. Look for repeated webbing at these little nooks (how's that for a scientific term?).
Finally, you could dig it up and make sure there is nothing underneath the plant like some old cement or huge rock, etc. A long shot but it has happened to me.

The tree growing near is a Redbud.
We had a pretty dry summer so maybe that was it. I have Helleboros growing near also and they are doing fine but the newer Clethra is having a hard time.