Locking In Read Committed With Row Versioning Isolation Level

I have a question on locking pattern of read committed with snapshot isolation level that when two transaction update two different records then why do they block to each other even if they have previous committed value (old version of record).

I executed the below batch from a query window in SSMS

--Session 1:
use adventureworks
create table marbles (id int primary key, color char(5))
insert marbles values(1, 'Black')
insert marbles values(2, 'White')
alter database adventureworks set read_committed_snapshot on
set transaction isolation level read committed
begin tran
update marbles set color = 'Black' where color = 'White'

--commit tran

Before committing the first transaction I executed below query from second query window in SSMS

--Session 2:
use adventureworks
set transaction isolation level read committed
begin tran
update marbles set color = 'White' where color = 'Black'
commit tran


Here the first session blocks to second session. These same transactions execute simultaneuosly in snapshot isolation level. So my question is why this blocking is required in read committed with snapshot isolation level?

Thanks & Regards,
Subhash Chandra