Blood Assasin node should improve Marrow Shards’ base crit chance and multiplier.
However it seems to improve crit chance and multiplier of bone splinters, not the actual shard.
I did few quick tests on dummy with 884% physical/spell crit chance in gear/passives:
Test 1 - marrow shards hitting dummy, 3/3 points in blood assasin
Test 2 - marrow shards splinters hitting dummy, 3/3 points in blood assasin
Test 3 - marrow shards hitting dummy, 0/3 points in blood assasin
Test 4 - marrow shards splinters hitting dummy, 0/3 points in blood assasin
Results:
test 1: 72 crits/min
test 2: 128 crits/min
test 3: 76 crits/min
test 4: 80 crits/min
These were short tests so there is some margin of error but difference seems quite noticeable.