If it turns out that there is no lore on gloomblades whatsoever outside of the AL adventures I've already referenced in the question, then it seems as though the lore presented in DDAL06-02 The Redemption of Kelvan is the only official source that contains any lore (since DDEP06-02 Return to White Plume Mountain just references the events of the other module, containing no additional lore).
The module states that a gloomblades are
perverted mirrors of the elven moonblades. Longswords made from a piece of frozen darkness brought from the Shadowfell [...] The sword is likely tainted by evil — reflective of Shar’s mission to unmake the world.
-- DDAL06-02 The Redemption of Kelvan, pg. 5
It also states that
the magical gloomblade, [is] a sword blessed by Shar with powers over shadow and time
-- DDAL06-02 The Redemption of Kelvan, pg. 3
And on handling the blade:
Any character picking up the sword feels a deep sense of despair and emptiness, as if all hope has drained from them. It is horrible to carry and no living soul would be able to bear it for more than a few days. The sword has no magical abilities that the characters can access at this time.
-- DDAL06-02 The Redemption of Kelvan, pg. 12
So, in summary,
gloomblades are longswords made from pieces of frozen darkness from the Shadowfell, likely tainted by evil because of the fact that they are "blessed" by Shar, although that it says "likely" implies that they might not be, though the implication is that they probably are evil. They resemble moonblades, but given that they are made from darkness from the Shadowfell, likely are not actually corrupted moonblades, but are rather created to resemble one. Gloomblades are horrible to carry, unbearable for more than a few days, since they make you feel a deep sense of despair and emptiness, as if all hope has drained from you.
At least, that's my take on the information presented in DDAL06-02 The Redemption of Kelvan.
Also, interestingly, since it seems that they aren't actual moonblades (since they were made from frozen darkness, etc), they might not actually be sentient like moonblades are, since the module doesn't explicitly say that they are, but now we're into speculation, so I'll stop here.