If you look closely at the particles that appear when you dig blocks, they appear to be semi transparent, as you can see the cracking animation through the particles. The particles that appear when the block has completely "been dug" seem to be opaque. I do not remember this occurrence on any previous version of Minecraft. I took a screen with the default texture pack to show that it is not the texture pack's fault. Please ignore the "Minecraft wingdings", that is another bug that has been reported earlier.