In baseball, it's not unusual for a promising rookie hurler to struggle for years until he puts it all together and turns into Sandy Koufax; it happens in music too, except instead of Koufax, you get Monty Alexander. About five years ago, Alexander graduated from being a glittery, facile pianist to a full-fledged, no-doubt-about-it keyboard ace; his control improved, he came to realize speed (which he can parlay with the best) wasn't everything, and his considerable technique came to serve rather than undermine the larger issues of the music at hand. These days, even a conventional standard can be converted into something special as Alexander stirs up his unexpected stew--the glassy virtuosity gleaned from Art Tatum, Count Basle's easygoing smoothness, and the hairpin turns of bebop, with the eruptive lilt of Alexander's native Jamaica added for spice. His sidemen are both offspring of other sidemen: Duffy (son of bassist Chubby) Jackson on drums, and Charnett (son of drummer Charles) Moffett, with his debut Blue Note album just released, on bass. In other words, Alexander has brought a trio that can not only match but challenge his rhythmic invention, which bodes well for the weekend. Tonight through Sunday, Joe Segal's Jazz Showcase, Blackstone Hotel, 636 S. Michigan; 427-4300.