business - In ArcGIS how can I append the number of points that are located inside each individual block groups of the entire US?

I am using ArcGIS 10 and I have two shapefiles; (a) a polygon shapefile of block groups of the entire US, (b) a point shapefile of customer locations throughout the US.

What I want to know if how many points (b) are within each block group (a). What tool should I used to add a new field to the block group shapefile that will count and show the total number of points located inside each individual block group?

I also would like to show that number as a percentage of the total number of points.