^-^

Initially you asked for an option of the burn brush tool which is a pixel editor. Alternatively you can use a vector shape, it is easier to handle, non-destructive and can be used to darken an area or as masking element. As far I know in V2 are some more options for nested vector objects / for adjustment masks in the workflow mentioned by @NotMyFault. The V1 examples below use colour + opacity + blend mode 'multiply' only, they could get combined with adjustment layers.

Alternatively, use a non-destructive workflow:
add a curves adjustment (or hsl or levels or exposure …) adjust curves to achieve the burn effect you want. It is applied globally but invert the inherent mask use any suitable brush, color, opacity, hardness and paint over mask to apply effect where you want it  the big advantage is you can adjust the strength of the effect any time

You're welcome.
We might be able to say more if you created a new document, dragged your arrow asset onto it, and gave us a screenshot showing the arrow selected, and including the Layers panel.

Hi @^-^,
Firstly, although you can technically search using 'hashtags', we recommend using the Tags function of the forums as these are clickable hyperlinks from each post, which take you to a list of other threads that use the same Tags - therefore are more useful for grouping relevant topics.
Secondly, unfortunately there is no keyboard shortcut support for toggling these options in the Develop Persona, you will have to use the icons from the top toolbar to enable/disable these as required, my apologies.
I hope this clears things up!

Same in macOS.

I'm unsure if this what you want, but in a levels adjustment (live or not) hold the alt key (windows) while changing the black & white level sliders - the blown out areas are then highlighted.

Cmd+tab is an OS-level shortcut to move to the next open app, and cmd+shift+tab moves to the previous open app.
Palatino appears to be asking for a keyboard shortcut to move back and forth between open documents (when open as tabs) in Affinity apps. I was not aware that there was a way to do so in Affinity apps, but Walt's clue helped me find it. It is called "Switch to Next View" and "Switch to Previous View" under the Miscellaneous shortcuts, and the default shortcut is cmd+` (that's a tilde) and cmd+shift+`. 

Via the Preferences dialog, in the section for Keyboard Shortcuts.
https://affinity.help/photo/English.lproj/pages/Workspace/customizingShortcuts.html

Pretty sure that's a bug which has already been reported.
 

@^-^ As is stated by Kaffeepause the safe mode may help. I keep forgetting about that. I have no experience with the utility he mentions so I can't comment on that.

Maybe try this:
In Finder, select one of your *.R3D files. Use File > Get Info to open the info window. In the Open with section, choose whatever app you want to be the default for that filetype, then click the "Change All..." button.
If that does not work, the Launch Services database may need to be rebuilt (although it is unusual for this to be necessary in recent macOS versions). More info about the Launch Services database & links to how to rebuild it can be found in various places on the web, like here.

I also would recommend to do so as @R C-R described.
As additional step you could start in safe mode which clears up some caches. How to use safe mode is described here.
If safe mode doesn't help, it also may help to clear the icon services cache of mac OS. Some time ago I had a similar problem and got mac OS back to work with a paid tool called TinkerTool System. Here is the site of the manual, which describes what it means to clear icon caches. Please scroll down to the section "Icon Caches".

@Bob Grahame You can get utilities that will do this for you on the fly, temporarily displaying them as an overlay on your screen.  I'm on a Mac and recently/finally bought Ergonis' KeyCue.  Here's an example showing my current shortcuts in beta 1.7.0.145.  Note that this includes a few that I have defined myself and should not be regarded as the default set.  Note also that what KeyCue displays is interactive, not just a static graphic that you see here.
Still on Macs, there is at least one other utility that you could try which is free: CheatSheet, from Media Atelier.
I'd be surprised if there wasn't something similar available for Windows users.

Watched this tutorial ?
 
 

shouldnt the sky be almost black & the green leaves white with infrared photography?   for example, with konica film stock
ah, ok, that was a color demo
converting to b&w by adding an extra adjustment layer will give the photo an infrared look of b&w film stock
layer>adjustment layer>black&white
red=300%
cyan=0%
blue=140%
 

I have produced a pdf version of the Affinity Photo manual that you can read without starting the program. It can be interesting to put it on your phone or tablet to read when you have some time to spare. Since the file is too big to upload here, I have put it on Dropbox for you to download. It is current for the last version of Affinity Photo. Comments welcomed. Here is the link:
 
https://dropbox.com/s/d3uxmmypdx9ruzw/Affinity%20Photo%20Manual.pdf?dl=0