Lee S. Roberts' Blue Moon is a standard early-to-mid 20th-century salon miniature. It is characterized by its lush, warm harmonies and simple, effective emotional appeal.
Focus on bringing out the primary melody clearly against the broken-chord left hand accompaniment.
Comparable difficulty and character — good candidates for what to learn next.
Blue Moon is rated Henle level 3 of 9 — late beginner repertoire, roughly ABRSM grades 4–5. In the RCM system it corresponds to about level 4.
You should be comfortable at Henle level 3 — meaning you can already play level 2–3 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of Blue Moon lasts about 3 minutes.
Blue Moon is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.
Pieces at a comparable difficulty and character include Believe Me, If All those Endearing Young Charms by Irish Folk Music, Valse sentimentale, No. 6 by Pyotr Tchaikovsky, Canson pel cabalet by Déodat de Séverac — see the full similar-pieces list on this page.