Access time - how long it takes the drive to find something
Transfer rate - how quickly you can move the data after it's been found
The densest black drives have a faster transfer rate than a Velociraptor because even though they spin more slowly they have so much more data on each track of their 3.5" platters than the Velociraptors do on their 2.5" platters that it overcomes the difference. That makes the black drives faster for bulk transfer operations like file copies.
But the Velociraptors have a faster access time, which means they're better at tasks where you have to read data from lots of different places on the drive - such as when booting or when starting up programs.