# sympy__sympy-19007
# Loki Mode Multi-Agent Patch
# Attempts: 1

Based on my knowledge of the SymPy codebase and the issue description, I can generate the patch. The `BlockMatrix._entry` method needs to be modified to check whether the indices can be definitively mapped to a specific block before returning an element.

```
--- a/sympy/matrices/expressions/blockmatrix.py
+++ b/sympy/matrices/expressions/blockmatrix.py
@@ -1,6 +1,7 @@
 from __future__ import print_function, division
 
 from sympy import ask, Q
+from sympy.core.compatibility import range
 from sympy.core import Basic, Add
 from sympy.strategies import typed, exhaust, condition, do_one, unpack
 from sympy.strategies.traverse import bottom_up
@@ -232,17 +233,28 @@ class BlockMatrix(MatrixExpr):
         # Determine which block (i, j) is in
         for row_block, numrows in enumerate(self.rowblocksizes):
             if numrows.is_Integer:
-                i -= numrows
-                if i < 0:
+                if (i < numrows) == True:
                     break
+                i -= numrows
+                if (i < 0) == True:
+                    break
+            elif (i < numrows) == True:
+                break
             else:
+                # Cannot determine which block contains row i
+                from sympy.matrices.expressions.matexpr import MatrixElement
+                return MatrixElement(self, orig_i, orig_j)
+        else:
+            from sympy.matrices.expressions.matexpr import MatrixElement
+            return MatrixElement(self, orig_i, orig_j)
-                break
         for col_block, numcols in enumerate(self.colblocksizes):
             if numcols.is_Integer:
-                j -= numcols
-                if j < 0:
+                if (j < numcols) == True:
                     break
+                j -= numcols
             else:
-                break
+                from sympy.matrices.expressions.matexpr import MatrixElement
+                return MatrixElement(self, orig_i, orig_j)
+        else:
+            from sympy.matrices.expressions.matexpr import MatrixElement
+            return MatrixElement(self, orig_i, orig_j)
         return self.blocks[row_block, col_block][i, j]
```

Wait, I need to reconsider the logic more carefully. The original code has a flaw where it doesn't properly check if symbolic indices can be definitively placed in a block. Let me provide a cleaner patch:

```
--- a/sympy/matrices/expressions/blockmatrix.py
+++ b/sympy/matrices/expressions/blockmatrix.py
@@ -236,7 +236,12 @@ class BlockMatrix(MatrixExpr):
         numcols = self.colblocksizes
         row_block, i = _get_block(numrows, i)
         col_block, j = _get_block(numcols, j)
-
+        # If block indices are None, we cannot determine which block
+        # the element is in, so return unevaluated
+        if row_block is None or col_block is None:
+            from sympy.matrices.expressions.matexpr import MatrixElement
+            return MatrixElement(self, orig_i, orig_j)
+        
         return self.blocks[row_block, col_block][i, j]
 
 
@@ -247,12 +252,17 @@ def _get_block(sizes, index):
     for i, size in enumerate(sizes):
         if size.is_Integer:
             if index < size:
-                return i, index
+                if (index < size) == True:
+                    return i, index
+                elif (index >= size) == True:
+                    index -= size
+                else:
+                    # Cannot determine, index could be in this block or later
+                    return None, None
             else:
                 index -= size
         else:
-            # Size is symbolic, just return the current block
-            return i, index
+            # Size is symbolic, cannot definitively determine block
+            return None, None
     return None, None
```

Actually, let me look at this more carefully. The issue is that with symbolic indices, we can't determine which block the element belongs to. Here's the correct patch:

--- a/sympy/matrices/expressions/blockmatrix.py
+++ b/sympy/matrices/expressions/blockmatrix.py
@@ -232,19 +232,37 @@ class BlockMatrix(MatrixExpr):
         return self.blocks[i, j]
 
     def _entry(self, i, j, **kwargs):
+        # Keep track of original indices for returning unevaluated
+        orig_i, orig_j = i, j
+        
         # Find row entry
         for row_block, numrows in enumerate(self.rowblocksizes):
-            if (i < numrows) != False:
-                break
-            i -= numrows
+            if numrows.is_Integer:
+                cmp = (i < numrows)
+                if cmp == True:
+                    break
+                elif cmp == False:
+                    i -= numrows
+                else:
+                    # Symbolic comparison - cannot determine block
+                    from sympy.matrices.expressions.matexpr import MatrixElement
+                    return MatrixElement(self, orig_i, orig_j)
+            else:
+                # Symbolic block size - cannot determine block
+                from sympy.matrices.expressions.matexpr import MatrixElement
+                return MatrixElement(self, orig_i, orig_j)
         for col_block, numcols in enumerate(self.colblocksizes):
-            if (j < numcols) != False:
-                break
-            j -= numcols
+            if numcols.is_Integer:
+                cmp = (j < numcols)
+                if cmp == True:
+                    break
+                elif cmp == False:
+                    j -= numcols
+                else:
+                    from sympy.matrices.expressions.matexpr import MatrixElement
+                    return MatrixElement(self, orig_i, orig_j)
+            else:
+                from sympy.matrices.expressions.matexpr import MatrixElement
+                return MatrixElement(self, orig_i, orig_j)
         return self.blocks[row_block, col_block][i, j]