What do the branches look like?

There used to be web pages up that had a lot to say about them, said no true Citrus was hardy below something like 28 degrees F. (That was before I noticed Poncirus being listed as a Citrus).

I look at stock of "hardy" types presented here and they look completely unsuited to northern conditions, with tender new growth being made quite late in the year. And the same vivid yellow green, hot climate plant foliage color as overtly tropical kinds such as oranges and grapefruit.

Nevertheless a friend has seen a sizable (~10 ft.) Lemon Tree on a former garden enthusiast property near the beach on Bainbridge Island in recent years. However this kind of location will be likely to have a milder winter regime than in the Portland area. As in nearly northern coastal California mild, in fact. Particularly if your site is exposed to continental winds coming down the Gorge during winter you should expect to be having to protect plants of this nature there.

the branches still look green and firm for the most part but the tips and some whole branches look blackish and sick.

Might come back from farther down. Moving it indoors (preferably to a bright greenhouse) would probably do a lot to help with this.

Low SE PDX temp here of +16.3 and several others right near 20 degrees would severely harm citrus left/planted outdoors. I'm east of you in the 40's (Avenues). You will have to judge how many year's growth your plant will lose, compared to getting a new plant. Early June should be conclusive.