I don't know if you were present for the lead-up to Minecon Earth and the initial plans for 1.13. The original plan was referred to as the Technically Updated update and it literally broke all of Minecraft. The command system was entirely redone, the way NBT/metadata was stored for items and blocks was redone, and a whole lot of bugfixes, optimizations, and other contentless changes were implemented. This either went extremely right (freeing up a whole lot of time they had to suddenly fill) or horribly wrong (too many changes, not enough time to make them all), because they then changed the plan by merging Technically Updated with Update Aquatic (which was slated as a 1.14 release). Anywho, with regards to the Technically Updated stuff, they basically got rid of block ids and essentially created a limitless system. Now, it's probably not actually limitless and perhaps they didn't even extend the limit by all that much. But they more than likely radically changed how the data for each block was organized/stored and that's actually what gives us the "essentially limitless" claim.
Here's the thing, though, even though they touted this as nearly limitless they've since approached creation of new blocks in basically the same way as before, when they had to actually live with hard limits. They haven't really added more than a handful of new blocks, and for the next 4-5 version cycles they probably still won't have added all that many. Doesn't make much sense, right? This leads me to believe that they're going to drastically change the definition of what a block is so that there's no more reason to bloat the palette with endless options (this is a common complaint in the modded community). An implementation similar in concept (if not execution) to the Carpenter's Blocks/Blockcraftery mods where they only define block shapes rather than shape-and-texture combinations would hugely reduce the palette size BUT would allow for a freer application of textures...in other words, you could define 1 shape block and get something like 150 block-and-shape combinations as a result. If this is what Mojang is actually aiming to accomplish, it really doesn't make much sense for them to spend time now to set up all the many shape-and-texture definitions they COULD do when in the near future those will "suddenly spring into existence" when that part of the core system is implemented.
1.14 was going to bring update aquatic and 1.15, hinted at by Jeb, would be a fan based update where hot suggestions in the community would be taken and considered. With all the new technical stuff being combined with update aquatic in 1.13, this supposed fan based update would then be 1.14. Other posts have suggested that Mojang has been looking at some of the most popular posts on the Minecraft suggestions subreddit. Different kinds of slabs and stairs have been longed for for as long as I can remember. It's possible that we could see them in 1.14.
I could understand not making it easy for creation of obsidian items. Maybe a diamond chisel would need to be created to carve out stairs or slabs. Im actually not interested in armor or weapons I just would like to use the black block as an alternative to cotton or other colored blocks that in theory wouldn't be as strong for building purposes.