What could be wrong with an Ethernet port on a black macbook?
You would normally expect better performance from a wired connection, even it it is slower on paper, so there would seem to be a problem, and it would be anywhere on the physical connection from the macbook to the internet router.  I would first check what sort of connection you have, 10 half duplex would be bad, 100 full better (1Gb full best, but I don't know if the blackbooks had gigabit ethernet).  Check this out on your switch/router and your macbook if you can.  It could be set/requested at either end of the connection.  A bad cable could force a slower auto-negotiated rate too, so try swapping it out.