I don't think the sentences are correctly punctuated because, unlike "it," "this" cannot merely work as an anticipatory subject (introducing the real infinitival subject that comes at the end). The clause starting with "this" is a complete sentence. If you want to clarify what "this" refers to, you should make the infinitival clause an apposition. A comma could help, as you suggest (but I'm not sure it's the best solution):
a'. This is a terrible thing, not to be able to see him.
b'. This is a bad blow to me, not to be able to see him.
It would be ideal to use "it":
a''. It is a terrible thing not to be able to see him.
b''. It is a bad blow to me not to be able to see him.
a'''. This question of not being able to see him is a terrible thing.
b'''. The impossibility to see him is a bad blow to me.