The lines "The old moon is tarnished / With smoke on the flood din" suggest a dim or obscured moon, potentially due to atmospheric conditions or pollution. The imagery of "smoke on the flood din" implies a hazy or smoky atmosphere.
The lines "The dead leaves are varnished / With color like blood" evoke the image of leaves having a deep red or blood-like color, possibly indicating the autumn season when leaves often change color and fall.
So, based on the last two lines, the season of the year indicated is likely autumn.