It is known from only four specimens and is endemic to a small area in northeast Sumatra, Indonesia, inhabiting primary lowland forest.

[3]: 14 The specimens have also been considered as being an aberrant form of the pale blue flycatcher (Cyornis unicolor).

The female has rufous-brown plumage, strong rufous lores, orange-rufous breast and a whitish belly.

In the Robinson & Kloss specimens, the immature male has brown-spotted buff plumage with a rufous breast and a whitish center on underparts.

[3]: 15 [4][6] Rück's blue flycatcher is endemic to Sumatra, Indonesia, around Medan, and presumably has a low population density.

[note 1] Last recorded in 1918, and due to ongoing habitat loss, poor surveying, and its small population and limited range, Rück's blue flycatcher is evaluated as "Critically Endangered" on the IUCN Red List.

[6] Rück's blue flycatcher might have been affected by the 2004 Indian Ocean tsunami,[10]: 19  and is also predicted to be extinct with 80% confidence.