Found fake blocks (disappeared after discovered)

I was playing and in the corner of my eye was a black concrete rectangle with shades of concrete powder that wasn't in the block menu after taking a screenshot I put a tall tower next to it I had a nap went back on only the tower was there for about an hour I searched for it came across another one (different) only to find out this time it was right next to my house I took a screen shot went onto photos showed them to my step dad went back onto Minecraft gone again they both haven't shown up on the Minecraft maps