Blue Monday is a blues-infused piano work by George Gershwin originally composed for an operatic scene but often transcribed for piano. It is distinctive for its early jazz syncopations and authentic use of blues blue notes.
The main technical hurdle is executing the jazz-style syncopation with rhythmic precision against the left-hand pulse.
Comparable difficulty and character — good candidates for what to learn next.
Blue Monday is rated Henle level 4 of 9 — intermediate repertoire, roughly ABRSM grades 5–6. In the RCM system it corresponds to about level 6.
You should be comfortable at Henle level 4 — meaning you can already play level 3–4 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 Monday lasts about 4 minutes.
Blue Monday 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 'S Wonderful by George Gershwin, Alexander's Ragtime Band by Irving Berlin, 12 Variations on "Ah vous dirais-je, Maman" by Wolfgang Amadeus Mozart — see the full similar-pieces list on this page.