Can I use 8 AWG THHN in conduit for a 50A outlet in my garage?
First, you are required to use the 60 degree table for household wiring below 100A. That will call out #6 copper or #4 aluminum wirewhich is 55A but you are allowed to round up.  Yhis is a wobbler point where I wouldn't normally mention aluminum (it is cheaper) except for the asterisk below. 

Your short 47A peaks don't inherently require a 125% (80%) derate for continuous load, but you didn't mention what your load was, and certain loads have a statutory requirement to be derated as a continuous load, e.g. Heaters.  The load's sly choice of 47A for its peak suggests the builder knows it is one. Regardless, it does not matter to wiring since 6Cu/4Al wire good for 50A is also good for 60A.  

Using two blacks for two hots is fine.  

Your plan to re-mark the wires has a problem: in THHN you are not allowed to re-mark to change a wire's function At all, not even whites to hots, unless the wire is 4 AWG or larger in which case you can mark anything you please.  And now you know why I mentioned aluminum. 

#4 will just fit inside that 1" conduit, though it will be a challenging pull. 

However, "bare" is a wire color.  You could strip all the insulation off a white to make it a bare wire.   Just don't get caught.   And be careful, if the wire is too finely stranded, it may go all rats-nest on you once out of the insulation, and that would not be acceptable.  

You are not required to use a #6 ground wire. For 60A, #10 will suffice.