The elm cultivar Ulmus 'Nigrescens' was identified by Pynaert

Edouard-Christophe Pynaert

Edouard-Christophe Pynaert was a Belgian botanist and horticulturalist who was a native of Ghent. He was a specialist in the field of pomology....

 http://asaweb.huh.harvard.edu:8080/databases/botanists?id=170401 as Ulmus campestris betulaefolia nigrescens. Considered by Green to be "probably Ulmus carpinifolia (: minor), but said to have been raised from seed of Purpurea".

Description

The tree was described as having leaves like those of the Birch

Birch

Birch is a tree or shrub of the genus Betula , in the family Betulaceae, closely related to the beech/oak family, Fagaceae. The Betula genus contains 30–60 known taxa...

, and with a darker and more persistent hue than the Wych Elm

Wych Elm

Ulmus glabra, the Wych elm or Scots elm, has the widest range of the European elm species, from Ireland eastwards to the Urals, and from the Arctic Circle south to the mountains of the Peloponnese in Greece; it is also found in Iran...

 cultivar 'Purpurea'.

Synonymy

NB. Nigricans: Bailey & Bailey, Hortus 625, 1930, as U. (americana) nigricans, name in synonymy for U. americana f. nigrescens, not U. campestris betulifoliae nigrescens Pynaert.