It may very well be helping her, and it's (probably) very helpful for you to have something to do during these stressful times.
I presume when you say "directly into her ear", you mean that her ear is perpendicular to your mouth. If this is the case, you have nothing to worry about. The decibels of her crying are likely to be higher than your singing, and no colicky child ever cried themself into deafness.
Noise-induced hearing loss [NIHL] can result from a one-time exposure to a very loud sound (at or above 120 decibels), blast, impulse, or by listening to loud sounds (at or above 85 decibels) over an extended period. The louder the sound, the shorter the time period before hearing damage occurs. - CDC
Normal conversational speech noise level is about 60dB, and will not cause
NIHL regardless of the duration of exposure. A battery-powered pencil sharpener, coming in at about 71 dB is reaching near to the upper limit of sound that will not cause any damage regardless of duration. (I would qualify this by saying that if you put your ear directly over the noisiest part and keep it there for a year, well, I'd not bet that it wouldn't cause any damage at all. I assume, though, that your singing is much less noisy than a pencil sharpener, and much more soothing.
Premature infants are more susceptible to NIHL than infants born at 38+ weeks, and some hearing loss from continuous exposure to sound in a Neonatal Intensive Care Unit has been documented, resulting in a call for noise to be 45dB or softer for preemies. Interestingly, in utero high-level noise exposure has been found to damage hearing.
Much more info can be found here: CDC: About Hearing Loss
Noise: A Hazard for the Fetus and Newborn