"These departures follow a post-merger reorganisation of the group's internal reporting lines and a number of senior management resignations," S&P fund analyst Justine Gorman said.
"Although S&P was comfortable that both BlackRock and ex-Barclays Global Investors investment strategies would largely be unaffected by these initial changes, given that the core of the investment experience remained in place and staff lock-ins had been negotiated, today's announcement has changed our view."
Jackson will work out his three-month notice with the group, while Walsh will leave at the end of June. They did not give a reason for their departure from the group.
Two of BlackRock's San Francisco portfolio managers will come to Australia to manage the scientific active equity strategies until a longer-term solution is found.
S&P has also placed the BlackRock index funds on hold until it has met with the team and the key individuals now managing these strategies.
The indexation team lost earlier portfolio managers Glen Harris and Nick Burt, although two of BlackRock's Melbourne-based managers, Ritchard Longmire and Andre Roberts, have been added to the group.
