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?