This is most likely a post operative effect similar to a bruise, with, indeed as you say, a little fluid collection. However, six weeks is a long time for things to resolve post operatively.
This warrants a call to the surgeons office, report this and ask them if you need to come in.
dt
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In some cases pigment changes in the skin after some procedures such as dermabrasion can be perment. Most acutal "stains" should sluff of as skin itself remodles. Before bleeching you might ask your surgeon if any compounds such as alphs glycolic acids [which help remove old skin] might be helpful to try first.

Goot Luck
dt