Hi ,
Blockage layer are used when u dont want u use a particular layer in specified area..
Advantage:
U can avoid shorts and avoid the extra parasitics..
Example: If somewhere clock is routed and u dont want it to couple with higher or lower metal then u use blockage layer ...
on my own understanding, maybe on the layer map layer1 is assigned for via1Blockage layer, then layer2 is for via1 layer. the minimum spacing between two via1 is equal to 0.22