I changed the code to Serial.write, but that doesn't seem to work either
That means there is something wrong as well, don't change it back.
does send MIDI, the MIDI IN led blinks
No you can't say that. The blinking MIDI IN LED means that some serial data has been received it does not mean it is in the MIDI format. There are lots of things that could be going wrong with it.
Get yourself a copy of a MIDI Monitor program and see what you are receiving.