hmmmm interesting . . . since the voltage out seems to be low, probably means that the sensor isnt getting enough light.
ok a few things to try:
1) shine a bright light on the sensor and see how how the value goes up . . .
2) make sure the infrared LED is connected properly - they are bipolar, so the long end needs to be connected to the 5V (stupid check
3) what kind of black line do you have? just because it is black in visible light doesnt mean it is black in IR. in the datasheet, read page 4 where it talks about 'Black on white paper.' Drawing ink will cause the results you have just described. try different black objects and see what happens. (for example, in IR, snow is black.)
4) theres a small chance the IR emitter is broken - cover up just the emitter with black electrical tape and check the voltage output
5) try a 50kohm resistor and check the outputs