Who do I call to pick up my IFR Clearance at KBKL?
When there is no separate Clearance Delivery frequency, just call on the ground control frequency to get your clearance.  

Separate Clearance Delivery frequencies are only needed at busy airports to relieve radio congestion and work load. 

Even at a "busy" airport sometimes you will see(hear) one controller work both Ground and Clnc Del when the workload is low.