A mandrel (20, 30) defines a void geometry in a layup for a blade shell (73) via a rigid mandrel core (22, 32) and a plurality of expandable bladders (24A-F, 34A-F) attached adjacently to each other in sequence around a periphery of the core, forming an expandable outer cover on the mandrel. The bladders are controllably expandable (50, 50B, 60) individually or in proper subsets of all the bladders to adjust pressure distribution and mandrel position within the layup after closing a blade shell mold over the layup. The bladders may be transversely partitioned (104) at multiple spanwise (S) positions along the mandrel in order to define additional sets of adjustable bladders along the span of the mandrel.