The highlighted words "A black day will it be to somebody" best suggest that things will not go well for King Richard.
This is because the absence of the sun is typically associated with bad luck or misfortune. The sky's "frowning and lour upon our army" further reinforces this ominous atmosphere.
Richard's own statement that he wishes "these dewy tears were from the ground" suggests that he already senses some impending doom.
So, in this expression, the phrase "a black day" is a metaphorical way of indicating that something bad or ominous is going to happen. It implies that unfortunate events are looming, creating a sense of foreboding or impending doom for King Richard.