Blackbird was Apple's code name for a new line of PowerBooks based on the 68LC040 processor. (The LC version of the 68000 draws less power and has no FPU.) These PowerBooks introduced a full-sized keyboard (with 12 function keys), replaced the trackball with a trackpad, had a 640 x 480 screen, and even had stereo speakers built into the lid. They were also designed so they could later be upgraded with a PowerPC processor.
These were a big hit in the office since they were also the first Macs with built-in ethernet capability, using Apple's AAUI connector. In the field, users raved about the dual battery slots, allowing up to 4 hours from a charged pair of batteries.
The 520 had a 4-bit passive matrix display.