My little guy had a similar "bump" on the back of his shoulder, where the neck meets it - about 1-2 cm, discrete, slightly movable, faint bluish tinge to it. We had it evaluated, and it is an angioma - ds had to be scanned to make sure that there were no angiomas elsewhere in his body as that can be bad news. My personal opinion: get it checked. Isolated angiomas = fine ... angiomas in the chest/abd/pelvis, etc. = not fine.
Mama to A 8/05 and S 11/06