Thanks for the invitation, apm.
I am not a great app user on the iPhone but I do shoot a lot of images with mine, and I did spend a weekend at a workshop with Rad Drew - http://www.raddrewphotography.com
Rad is a fellow Hoosier and is a dedicated IPhonographer as are a couple friends of mine. The link to Rad's site offers a number of app suggestions especially for B&W conversions. His site also has an extensive list of apps he uses on his iPhone XS Max
One app I love and use very almost every day, is PhotoPills https://www.photopills.com
which is avalible for both iphones and android phones. It tells you the times of sunset, sunrise, moon phases, exposure tables, and helps plans shooting sites and times. For $9.99 it is a real bargain and I recommend it highly - for phone photography and regular photography.
Snapseed has been a mainstay for phone photogs for a very long time - it allows image selecting and editing on a small device like a phone - both IOS and Android. https://snapseed.online
I must confess that I prefer to do my image editing on a large Hi Res monitor - on my iMacPro - so I Air drop my iphone files to my desktop, and then import them into LightRoom and store them on my external RAID device, and then edit them in LR and or PS, or other image editors as needed, like Topaz, SilverFX, ColorFX etc.. Works well for me, but not very mobile. But maybe more precise and controllable for me.
Like Vilen, I use the native iPhone Photos editor for minor exposure and tonality edits to my images, and if I need more I will move on to Snapseed on my phone. A great app if you have little fingers or a big screen.
A few apps Rad mentions that he uses frequently, in addition to the ones I mentioned above, include Dramatic Black and White, Noir, Simply B&W, Provoke, Image Blender, Vintage Scene and others.
I should mention that there are two separate types of apps - some are editors or blenders that allow one to edit an already captured image. Others are apps that allow you to use and control the camera in your phone in a different manner than the basic photo app that comes with your phone. Provoke is a camera app, named after a B&W photo magazine in Japan in the 1960s.
I should also point out that I find I really do like the Portrait setting on the iPhone - the ability to capture a close subject and really blur the background is really a great trick that I find myself using often.
Darkroom looks very interesting - I will have to try it out, although it looks like it could easily cost $20 bucks of more in in app purchases. I think Darkroom has been upgraded to Darkroom 4.1 for iPads also so that might be very interesting - bigger screen to work with than a phone for those who prefer a larger screen image for precise editing