If we had an estimate of the number of copies of blockchains in existence, we could compute the weight.  See:

http://superuser.com/questions/11423/how-much-does-a-gigabyte-weigh-on-a-hard-disk

So for example, if 1GB is ~1 microgram, then a single blockchain would be 6.3 micrograms.

6.3 * 158,730 = 1,000,000.  So 158,730 blockchains would equal 1 gram.

edit: spel goode