Phal root rot?
 

Hi, one of my indoor orchid’s leaves have been yellowing with black patches where the leaf meets the stem. The very bottom leaf started yellowing first and is making its way up
so I’m wondering if the yellowing is natural or potential root rot (because of the black patches).

The oldest leaves will naturally eventually die off. As for whether bad roots are contributing, impossible to know without looking. When was the plant last repotted? Phals can be repotted any time, so pulling it out and observing what is happening in that pot is your next step.

I'm pretty new to this myself but one thing I noticed are water drops on some leaves. If your temperatures have dropped it could be that water is sitting in the joint between the leaves and the stem and causing a problem. Just a thought, I'll let the experts confirm/discuss whether this could be contributing.
Good luck, hope you get it sorted!

Thanks Roberta, I repotted the orchids last week. I cut out the dead roots but may have inadvertently left some rotting parts behind. What should I be looking for in terms of root rot when I am examining the root itself?

Fuerte Rav -it’s a perpetual trial and error for this novice orchid grower =) Good luck to you too!

Don't worry about bad ones you might have left behind. With the fresh mix, new roots will develop. It will just take awhile, orchids don't do anything fast. In the meantime, the old ones can still help the plant... at least lend stability for it in the pot (really important that it doesn't wobble) Now, just give it good care, don't worry about losing a couple of old leaves.

As for trial and error, EVERYBODY does that... learn from the failures as well as the successes (or maybe even more from the failures)

So I took the orchid out from its part and there are black parts where the roots meet the crown. Is this normal?

Roots look pretty good to me. Just put into fresh mix, which will encourage them to produce yet more. I don't see anything to worry about.

Looks typical to me too. Frequently, roots have taken a beating in box store Phalaenopsis and infections can start to set in if they’re kept too cool and wet which moss in winter can exacerbate. Either repot in moss and keep warm or switch to an airier media like bark and any issues the roots are having should resolve themselves.

UPDATE

I noticed the a root had turned brown so I took the phal out of its pot and all of the roots have become mushy except for one or two. Oh the horror. When I gently tug at it, the casing of the root comes off easily. Should I remove the mushy casings and let it dry out for a bit? The leaves are fine so I had no idea...also, when I took it out of its pot I saw the tiniest white bug. It disappeared before I could take a picture of it but wondering if I need to treat the roots with some sort of insecticide.

You can remove the mushy "casings" - the rotted velamin. Leave the stringy centers - along with still being able to (inefficiently) supply hydration to the plant, they'll help anchor the plant firmly in its new medium. If you rinse off the roots as you clean them up, you will likely remove any bugs that are hiding in there too. So not a need to give any treatment, just pot up in fresh medium...