Tablets were easy to find in the late '90s and early 2000's. I was going to say "popular" but they were obviously not popular enough to survive.
If I remember right, the Purex tabs sold around here were white, no blue speckles. I bought a box or two, they used to dissolve well enough to be practical in the dispenser drawers, unlike Surf, Wisk and Tide, which practically had to be smashed (still inside the plastic pouches) into powder unless you wanted to put the tablets inside the washer drum, as instructed in the boxes.
Wisk and Tide tablets were good enough to clean most stains by themselves, just like their other versions -- some people in fact compared the cleaning power of Wisk with Bleach Tablets to Persil Megaperls. Purex Tabs behaved more like powdered All, in that lots of stains would lift, but you had to add some form of enzyme product (Biz, Clorox 2) to the wash if you wanted better performance -- I don't remember how they compared to standard Purex because I have not tried it in over 20 years, sorry.
Lots of people got very miffed when tabs/tablets disappeared from the market, particularly people who lived in apartments and had to use laundromats, because it was much easier to bring a few pouches with your clothes than large bottles or big boxes of regular detergents.
The other nice thing about the tablets at that time was that it was harder to find HE detergents for the front-loading washers, but despite the fact that none of the tablets said they were HE detergents, they had suds that were controlled enough to be used in the frontloaders if one was careful enough.
In any case, by mid-2000's they basically disappeared from the market.
Cheers,
-- Paulo.