I'm not one to blend mic's and pickups. I think if you're happy with your live sound through an amplifier using a pickup, then that's all you need. The same applies to
recording microphones and pickups but in reverse. If there's a microphone that sounds great on your bass, then adding a pickup isn't really necessary. I understand that
lots of players benefit from blending the two. It just not a sound that I enjoy. Just my take however. When I first got my Grace Felix I thought " wow now I've got
the perfect setup to use both and I hardly ever do that.
Choosing a mic or pickup is often compared to the idea of choosing a brush when one is painting. IMHO blending a mic and pickup just expands the range of tones and textures you can create. Totally understandable if you don't find this extra range usable, but many do.
For upright I see "pickup VS mic" more as what works in a given situation. For live work I have experimented with mics a bit and always found them completely useless as I feel the bleed from other instruments is too much of a cost to pay. This is somewhat situational on the types of music and ensembles I play in. The ensembles tend to be big and loud. Also sending a mic through an amp at the volume level I typically require does not work. There is simply not enough gain before feedback to get the necessary SPL.
My impression is the Chuck Israel (CI) signature mic is probably about as good as it gets, as this mic is custom tuned specifically for close mic'ing an upright. The DPA4099 also sounds pretty darn good, but I don't think it would do as well in a live environment as the the CI. In pinch, I own several ATM350As that I believe perform fairly close to the DPA4099. The only bass specific mic I have experience with is the AMT S25B. It sounded fabulous when the bass was playing solo, but was useless with an amp or playing in a big band.
About the only place where I would consider mic'ing upright basses live would probably be an outdoor pop's concert. I have heard a few of these and they always sounded awful. The system was poorly chosen and I had the impression the audio techs were in over their heads. I am pretty confident that I could do better, but the right type of equipment is essential or it's a lost cause. Also it would be nice if the basses were not too close to the tympani. I have a great deal of experience mic'ing full concert bands, which are way louder than string ensembles. I started out mic'ing the tympani, but quickly found out that it is almost never necessary to give a dedicated mic to this instrument.
For studio work, I think I would most likely prefer 100% mic if I were playing upright through cans in an acoustic isolation module. I think a mic would also work with a jazz combo, if the bass was strategically located and surrounded by acoustic panels (gobos), to minimize bleed from the drums. Also it helps if the drummer plays with some restraint

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Regarding blending, sometimes it's nice to capture the drier, tighter, and deeper lows from a pickup and use the mic to capture a bit of that "authentic" upright tone. What would be best depends on a lot of factors such as the bass, pickup, style of music, etc. Never recorded on upright so my opinion is academic on this point.
I have recorded a bit on electric and I always preferred the straight DI sound over the mic'ed cab. But that's more about the type of tone I prefer than anything else. Of course it wasn't necessarily my decision

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There are lot's of recordings where I think the bass player's tone is awful, and yet it's perfect for both the song and recording. There are also plenty of examples of great bass tones captured either by mic'ing the cab or blending a mic with the DI.