GarageBand doesn't let me import a file [ipad]
GarageBand only supports those MP3 audio files which have a sample rate of 44.1 kilohertz (kHz) and a bit depth of 16 bits.

I guess you have to convert the MP3 files to GarageBand's more compatible AIFF file format. Use iTunes to convert MP3 to GarageBand more compatible AIFF format.

The following site could give you wider clearance: http://www.bigasoft.com/articles/mp3-to-garageband.html 

If you have all the required format as expected and still if it is giving an issue, I would suggest you check your project. It does this when your project has become too full. Try deleting tracks out of your project and then drag the audio file back into Garage Band and it should turn up blue