Finished

Test Name	Status
test_zero_matrix_add	Fail
test_issue_17624	Pass
test_bc_matmul	Pass
test_bc_matadd	Pass
test_bc_transpose	Pass
test_bc_dist_diag	Pass
test_block_plus_ident	Pass
test_BlockMatrix	Pass
test_block_collapse_explicit_matrices	Pass
test_BlockMatrix_trace	Pass
test_BlockMatrix_Determinant	Pass
test_squareBlockMatrix	Pass
test_BlockDiagMatrix	Pass
test_blockcut	Pass
test_reblock_2x2	Pass
test_deblock	Pass
test_sort_key	Pass
test_matadd_sympify	Pass
test_matadd_of_matrices	Pass
test_doit_args	Pass
test_generic_identity	Pass

     def _blockmul(self, other):
         if (isinstance(other, BlockMatrix) and
                 self.colblocksizes == other.rowblocksizes):
+            return BlockMatrix(self.blocks*other.blocks)
+            # Perform proper block matrix multiplication
+            # (A*B)[i,j] = sum_k A[i,k] * B[k,j]
+            n_rows, n_inner = self.blockshape
+            n_inner_other, n_cols = other.blockshape
+            new_blocks = []
+            for i in range(n_rows):
+                row_blocks = []
+                for j in range(n_cols):
+                    # Start with the first term to avoid scalar/matrix mixing
+                    block_sum = self.blocks[i, 0] * other.blocks[0, j]
+                    for k in range(1, n_inner):
+                        block_sum = block_sum + self.blocks[i, k] * other.blocks[k, j]
+                    row_blocks.append(block_sum)
+                new_blocks.append(row_blocks)
+            return BlockMatrix(new_blocks)
         return self * other

harrison