I've noticed black level issues lately on Prime, but not on Netflix on my recent Rokus. I'd be curious to know what client and TV you were using. I have a friend who watches Netflix in a browser interface, and he is constantly complaining about PQ. Knowing what I know about how Netflix builds the catalog they distribute to their CDN servers, most recent content should look really good if not great. I certainly wouldn't expect it to look worse than a Dish recording or live TV delivered over Sling. The exception being if the content provider gives Netflix a subpar original to work with. GIGO and all that.
OK, the WORST I had observed of awful black levels on Netflix was viewing through the last Gen TOP TIER FireTV--not those under-powdered sticks, and the CUBE is now the KING of Fire TV specs--with a 100mbps downstream (very reliable ISP and speed), running through Yamaha AX series AVR with QDEO chip to Sony Bravia (can't recall the model), but EXCELLENT PQ. This set up has presented breathtaking PQ--even better, the TV can display PAL (frame rate, of course)
natively for FLAWLESS PQ playback of several of my PAL frame rate DVD's and Blu-rays. Now the content:
I think the content plays a very big role in what Netflix may CHOOSE to send in highest quality it can (probably big movies, etc.) and other content. OK, the WORST was the US version of "The Killing" with extremely dark scene where the detective is carefully making her way through the dark in what appears to be a life and death situation in the very first episode and very first scene. This was a recent US TV show sometime about 2017, I think. VERY modern HD production and the black was just an awful display of pixels, and the only way to get it to look DECENT was to adjust so, but then the rest of the show was way, way, too dark to see most of the picture.
Let me make the observation that I believe both Netflix and Hulu seem to reserve high quality PQ for blockbusters--in general--and less viewed content is given barely adequate. Considering the finite amount of bandwidth on the backbone and at the edge with the ISP's, it makes sense for such a scheme. Dish has a similar practice where premium movie channels and some general interest (especially Disney Channel East's glorious PQ--Charlie and Bob (Iger) are buddies now) allowed great PQ and other lesser channels can look obviously lacking detail or even "color and contrast" data and looking far too soft.
The original UK version of
Shameless (far superior version to the US version ) Series 9 looked pretty bad on Hulu, and BARELY less pretty bad on Netflix. PQ was noisy, some grain and just plain NO DETAIL (way too soft or in low resolution). It looked to me that both services were sending out Standard Def versions. Well, I was looking to save money and watch the remaining series and episodes on either Netflix or Hulu, but both were so bad, I bought the DVD's (yes, SD DVD's) which was a far better PQ than Netflix and Hulu were streaming. However, some lesser known things such as "The Internet is Coming" the farewell episode of the IT crowd, looked very good on HD Hulu. Also,
The Thick of It Series 4 on Hulu is in good quality HD.
Look, there are some examples of great, good, and decent PQ on both N & H, but A LOT of other content may be given the low PQ treatment on the way out of the servers. I can say that Netflix and Hulu have gotten BETTER with consistent increasing quality of PQ.
I forgot to mention Amazon. IMHO, Amazon seems to be the PQ leader in just about EVERYTHING it streams. I can't recall a less than GOOD (often GREAT) PQ from Amazon that was NOT the fault of the source production. Then again, Amazon does have pretty much all the AWS it wants; Everybody else has to pay what it feels makes sense for its varied content. Also, I think sometimes the services may have to pay MORE for the better source when it comes to OLDER TV or Movies.