Darkness in underarms could be because of hormonal issues. It would be a wise decision to have a consultation to rule out underlying causes if any. Treatment ranges from medicated creams to chemical peels (in sessions), depending on underlying cause.
Laser hair reduction is recommended as the preferable method of unwanted hair, you may consider it, as shaving may leave to ingrown hair which could feel as bumps as you mentioned
Next Steps
Take appointment for online consult to discuss further treatment options
Health Tips
Consider weight reduction if you are obese