  Twisted current algebras are fixed point subalgebras of current algebras
under a finite group action. Special cases include equivariant map algebras and
twisted forms of current algebras. Their finite-dimensional simple modules fall
into two categories, those which factor through an evaluation map and those
which do not. We show that there are no nontrivial extensions between
finite-dimensional simple evaluation and non-evaluation modules. We then
compute extensions between any pair of finite-dimensional simple modules for
twisted current algebras, and use this information to determine the block
decomposition for the category. In the special case of twisted forms, this
decomposition can be described in terms of maps to the fundamental group of the
underlying root system.
