It is possible to say that Belgium had a hand in creating the system that led to the conflicts that have been funded by blood diamonds. However, we cannot say Belgian colonization is solely responsible. Belgium did not colonize all of the places that have blood diamond conflict and Africans themselves bear responsibility for their own actions.
Belgian colonization did, however, help cause this problem. The Belgian colonization of the Congo laid the foundations for failure of the colonies once they became independent. The Belgians created colonies with many rival ethnic groups who could be counted on to come in conflict once independent. The Belgians also did not educate their subjects well or give them any sort of governmental structure that would lead to stability after independence.
For these reasons, their former colonies fell into disorder after independence. The disorder led to conflicts of the sort that are fueled by blood diamonds.