It might be a little tough to summon up the blues on a Sunday afternoon. One hopes it'll at least be overcast when four locals - D. Mark Conway, A. J. Alenduff, Gideon Wainwright and Gordon Bonham - gather to play an installment in the Central Library's Hometown Roots Concert Series, 1:30 p.m. on Sept. 20 in the Clowes Auditorium, 40 E. St. Clair St. Of course, it's a good time and place for those who aren't old enough for the Slippery Noodle to learn about the blues. And like all Hometown Roots Series events - and just about every show at the library's Clowes Auditorium - admission is free. Bonham is perhaps the most celebrated of the crowd, having performed with Al Green, B. B. King and in myriad band and solo formations for decades. Newer arrivals to the scene, D. Mark Conway can be heard with the Naptown Swingin' String Band, A.J. Alenduff in The Will Woodrow Project and Crabtree's Trunk, and Gideon Wainwright with his own groups or all by his lonesome.