Better check when blockreshape is type unstable

Author: mtfishman

src/BlockArraysExtensions/BlockArraysExtensions.jl

@@ -278,7 +278,7 @@ function _blockreshape(a::AbstractArray, axes::Tuple{Vararg{AbstractUnitRange}})
     return reshape(reshaped_blocks_a[I], block_axes_I)
   end
   bs = Dict(Block(Tuple(I)) => f(I) for I in eachstoredindex(reshaped_blocks_a))
-  if !isconcretetype(valtype(bs))
+  if !isconcretetype(keytype(bs)) || !isconcretetype(valtype(bs))
     # This branch only seems to be required in Julia 1.10, not Julia 1.11.
     # TODO: Remove this branch once Julia 1.10 support is dropped.
     bs = Dict{Block{length(axes),Int},AbstractArray{eltype(a),length(axes)}}()