I take it this blue film is Ortho?
Sort of, yes. All x-ray film is virtually insensitive to red light. Furthermore, x-ray film falls down into two categories: blue-sensitive (only sensitive to blue light) and green sensitive (sensitive to both blue and green light). I used a green sensitive film here (Ektascan B/RA).
Can it be processed with Kodak D-76 and Kodak fixer?
Certainly. Any regular B&W film chemistry can be used. You'll have to figure out the proper development time for your film/developer combination. On the large format photography website you'll find a huge thread outlining experiences with various combinations. Unfortunately, there is no convenient overview of good combinations, so personal testing is in practice the best way to find out what works for you.
If I use it with a red filter on my lens, will it darken skys?
Uhm, well, a red filter with xray film will not only darken skies, it will darken everything and you will get virtually no image as the film is not sensitive to red light. With green-sensitive film, you could use a yellow filter to get some tone in skies, but you'll never get as pronounced an effect as with panchromatic film and orange/red filters. If you want deep dark skies, x-ray film is not a good choice.
$29/100 sheets of Fuji is a screaming deal!
For a reason, too - it's a pretty compromised product. It's fine for its intended purpose, but as photographers, we of course use it differently and that generally results in problems. There are two main issues to deal with:
1.: x-ray film comes in double-sided and single-sided flavors. The cheap Fuji film you find is most likely double sided, i.e. it has a sensitive emulsion on both the front and the backside (i.e. there is no front and backside; they're both the same). Since the emulsion scratches easily, this generally results in problems with scratches during processing. I know that several people get good results with double sided film, but I'm not one of them. I've never been able to work out a reliable process method allowing me to prevent all scratches. The best I could do is some scratches along the edges and in the corners of each sheet; the worst was scratches all over the place. I've given up on double sided film and consider it a waste of my time to keep experimenting with it (this is after several hundreds of sheets of trials). Furthermore, since the backside (facing away from the lens) is always a little bit out of focus and there is no antihalation layer in double sided film, the rear emulsion will always record a slightly blurred/defocused image. In contact printing, this is also the emulsion that is not in direct contact with the paper, and hence more blurriness results. Optimal sharpness simply cannot be expected with double sided film. Some find the results acceptable, but I don't. There are ways to deal with this such as stripping the backside emulsion after processing - which I tried, but it's messy, risky and just an overall chore/nuisance.
2.: to make matters worse, the emulsion on x-ray film is much softer than on regular photographic films. It'll scratch if you so much as point at it, particularly when it's wet. It's very hard to process a sheet of film without at least one side of it touching things such as trays etc. and with double sided film, this will inevitably result in scratches. Yes, I've tried everything from glass processing trays, sheets of glass on the bottom of trays, to agitation schemes that never let either side of the emulsion touch the bottom of the tray, but entirely scratch-free results proved to be virtually impossible to realize for me. Some people accept the odd scratch here and there or some scratches in the corners of their images, but personally, I don't. I want entirely scratch-free negatives. And that just makes double-sided film a less obvious choice.
The takeaway for me at least is that double sided film is finicky and for me, it's much more trouble than its worth. Fortunately, there is also single-sided x-ray film, which is usually intended for mammography applications. Most of it has the added benefit that it also features and antihalation layer, which helps prevent blooming highlights. It is usually more expensive than the omnipresent single sided stuff, but still cheaper than most photographic sheet film - although including shipping and taxes to Europe, Fomapan 8x10 film is roughly similar in cost per sheet for me as Ektascan B/RA.
Further challenges of x-ray film are its tendency to be very prone to building high contrast (which is actually a bit of a benefit when it comes to salt printing), it usually is more prone to mottling and uneven development (so your agitation scheme is much more demanding than for normal film, and you may have to experiment with prewetting and other techniques) and I find it also is prone to developing pinholes with certain developers if an acid stop bath is used (much like the photographic films of the 1950s). Essentially, x-ray film takes you back several decades in emulsion technology in various ways (although Ektascan does feature modern t-grain technology) and that means that it's just more finicky to process for high quality results. So yes, it's cheap, but it comes at a price at the same time.
If you're willing to invest many hours into experimenting and spending many sheets of film and liters of development on figuring out a consistent processing method that works for you within your requirements, it can be worthwhile. But if you're looking for a no-fuss solution that works more or less out of the box, just stick to regular photographic film.