Don't know if anyone mentioned this but as the title states i cant upgrade the nightingale blade. I go to the grindstone & it doesn't show up.

Do you have that perk that allows you to upgrade magical items ?

And even with perk, if it's already been raised to the limit of your BS abilities, you can try and get one of the Blacksmithing potions that will improve how much you can do to weapons and armor by whatever %. It worked for me a few times to upgrade an enchanted item.

The highest levelled version of the blade cannot be improved.

i have 100 smithing & all smithing perks plus apparel that equals 52% smithing upgrade & i just got the blade & went to windhelm grindstone, it didnt even come up in the menu to upgrade.... its at 37 base power i know that is not its max power!.... on my 1st playthrough im pretty sure i could upgrade it though...hmm... i dont have to have the nightingale bow do i? i wouldn't think so....

Yeah the highest leveled version wont show up and cant be upgraded. Had this happen to me on my first character as i was level 81 when i got it. Made a point to get it earlier on my second character and was able to improve it.

well i guess the nightingale blade goes in my house to rot... will this happen with the nightingale bow then?....