Why are jet engine fan blades painted? I came across this extremely interesting article with details on a 2018 United Airlines Boeing 777  engine failure, which is also related to the more widely-publicized failure a few days ago: http://aerossurance.com/safety-management/ndi-failures-b777-pw4077-fbo/ . Amongst others, it contains the following:

The records for the TAI [Thermal Acoustic Imaging] inspection in July 2015 as well as an earlier
TAI accomplished in March 2010 revealed a thermal indication in the
same location as where the LCF crack occurred. The records for the
fractured fan blade’s July 2015 TAI inspection was
annotated ‘paint’ that, according to the inspector, was consistent
with him accepting the indication because he thought it was an issue
with the paint.
Flaking paint was a regular issue, affecting perhaps 25% of blades,
requiring either a touch-up or a complete re-paint.

which begs the question: why are fan blades painted? The only reason I can think of is so that damage from small objects (or birds) sucked into the engine is easier to see? But, if I read the article correctly, the paint can also mask more serious issues - in this case, the inspector thought that the TAI indication was due to flaking paint, which is apparently common, when in fact it was a defect in the blade. So maybe it would make sense to leave the blades unpainted?
The final report notes:

At the time of each TAI [thermal acoustic imaging], the inspectors attributed the indication to a defect in the paint that was used during the TAI process and allowed the blade to continue the overhaul process and be returned to service. [emphasis mine]

The paint being referred to is used for the inspection. Its function is to improve the emissivity of the object.