Circular or oval, usually reddish, blister gall...Parallelodiplosis subtruncata (Felt)
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P. subtruncata. This species is in every active blister gall I have seen, so is evidently the gall maker. The galls are about 4 mm in diameter and are usually on the leaf laminas but may also occur on the veins and petiole...Hosts: C. florida, C. racemosa, C. stolonifera...

[Elsewhere discusses Lasioptera corni Felt, which was reported once from the same galls - concludes that it was likely not the gall inducer]

[Parallelodiplosis] subtruncata. USA (Idaho, Minnesota, Pennsylvania, New York, Massachusetts, Maryland). Orig. unk.; C. florida, C. stolonifera, C. sericea (Cornaceae).
Cecidomyia subtruncata Felt 1907a: 44. USA: New York, Albany; HT; ♂; NYSM.

IDAHO
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ex Cornus sericea...(5 adults, 2 pupal skins)
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These specimens are the basis of the records of Parallelodiplosis subtruncata from Idaho and Cornus sericea L. (Cornaceae) listed by Gagné and Jaschhof (2021); the known distribution of this species is otherwise from Minnesota to the east coast. At the time of collection in early autumn, some of the leaves had turned red or yellow, but the galls remained as large green spots (each centered on a major vein), making them quite conspicuous...

from leaf blister gall on Cornus sericea Linnaeus, June, 2009, Aspinwall