That's a region of recent star formation in the disk of NGC 4302. Regions of recent star formation include short-lived, high-mass O and B stars, which are blue (because they're hot) and luminous enough to outshine the (redder) lower-mass stars that are also present. If you look carefully at the image, you can see hints of blue light from other parts of the disk (e.g., in the upper right and upper left sides of the disk). You can also see lots of recent star formation in the disk of the neighboring galaxy, NGC 4298.
If you look at the full-resolution (84 MB) version of the image available here, you can see that the blue region resolves into many blue pinpricks in the midst of a fuzzier blue haze, a clear indication that it's made up of lots of bright blue stars (or small clusters of bright, blue stars) and a larger number of somewhat fainter blue stars. You can see the same thing going on in the blue regions of NGC 4298.
There are probably other, similar regions in the disk of NGC 4302; the reason this particular region shows up so well is probably because it's located in the outer part of the disk on the side closest to us, so there's relatively little dust blocking its light.
(Although there is undoubtedly ionized gas in the vicinity of those stars, the gas is not producing enough light to show up in the image -- you would in fact see a reddish/pinkish region if it was mostly light from ionized gas.)