Keyrunes as blockers

[Dane]

I was drafting last night, and went to use my Golgari Keyrune as a blocker. I activated it in the declare blockers step (which I BELIEVE is legal, but not 100% there...I know I've experienced some odd seeming timing issues with activations when using Forge,) and in response to my activating it, my opponent burned it before I could declare it as a blocker. I didn't think he would get priority to do so between activation and block declaration, but he was insistent that it could not be used as a blocker. What's the proper timing there?

[Keyeto]

Unfortunately, you are unable to do this. The last opportunity to make a creature available to block is before the end of the declare attackers step. The very first thing that happens in the declare blockers step is to declare blockers, and it's a turn based action that doesn't use the stack, so at that point it's too late to activate Keyrune, etc., to block.

[Pleeb]

I look at it this way. Declare an attack, cast spells, declare attackers, cast spells, declare blockers, cast spells, deal damage end of combat phase. After your opponent declares his attackers you have a chance to respond with affects like activate a keyrune. Before you can declare him as a blocker your opponent has a chance to respond by removing it or use some other means to prevent it from blocking.  Long story short, his creature is not blocked by the keyrune.