The most common and most likely answer to this crossword clue is the 5 letter word SMALT
S
M
A
L
T
Buddy explains!
The straight part
We know that "Deep blue" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case deep blue = smalt.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SMALT mean?
SMALT noun- A deep blue pigment made from powdered glass mixed with cobalt oxide