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 block matrix multiplication manually to avoid
+            # ZeroMatrix objects being converted to scalar Zero objects
+            from sympy.matrices.immutable import ImmutableDenseMatrix
+            rows, cols = self.blockshape[0], other.blockshape[1]
+            inner = self.blockshape[1]  # = other.blockshape[0]
+            new_blocks = []
+            for i in range(rows):
+                row_blocks = []
+                for j in range(cols):
+                    # Compute the (i,j) block as sum of products
+                    block_sum = None
+                    for k in range(inner):
+                        product = self.blocks[i, k] * other.blocks[k, j]
+                        if block_sum is None:
+                            block_sum = product
+                        else:
+                            block_sum = block_sum + product
+                    row_blocks.append(block_sum)
+                new_blocks.append(row_blocks)
+            return BlockMatrix(ImmutableDenseMatrix(new_blocks))
         return self * other

goodagent