What is the meaning of seismic spectral blueing?
It is a kind of spectral shaping, intended to increase the vertical resolution of seismic reflection data.

The logic goes like this:


Seismic data is band-limited and lacks high frequencies. This limits its vertical (travel time, and thus thickness) resolution. This is annoying because we often care about thin beds. 
The spectral peak of seismic data tends to be at a fairly low frequency compared to its total band. That is, the longer wavelengths are disproportionately strong compared to the short ones. Thinking of an analogy with the electromagnetic spectrum, we could say that the spectrum is reddish. 
We assume that geology is whitish, or scale free, so all 'frequencies' are represented (when we're in the time domain, they are frequencies; when in depth they are wavenumbers). 
So maybe we can reshape the seismic spectrum to more closely match the geological spectrum, as derived from logs, by boosting the higher seismic frequencies a bit.
One way to do this is to design an operator, a sort of wavelet, with which to convolve the seismic data. The result would be the seismic with the boosted frequencies. 


Here's one example of the application of the method [PDF] from Yadav et al. (2010).

It's just my opinion, but I'd treat the idea and the method with the same skepticism as any method claiming to enhance seismic resolution. That said, I'm sure there's a champion of blueing for every detractor. Caveat interpretor.