Do you mean something like this Dirty Don poster? I made this myself using a Michaels shadowbox frame. I couldn't find anything either. I hid the LEDs with wood corner molding that I glued inside of the shadow box.
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Here it is currently on the wall with semi bright lights on in the room to give it some better perspective with a camera.

Let me gather some pics and I will post more about it.

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I guess what would be a good question is if you want to light your room with black light, or just the poster (and a few things near it from the black light reflection). For now (?!?) I just want my poster to be lit, which is why I did it that way.

What you need:
1) 24x18 shadowbox frame (sometimes they put them on sale for 50% off) - https://www.michaels.com/walnut-shadow-box-by-studio-decor/M10546449.html

2) LED light strip with the 120 volt adapter - I got mine off of eBay.

3) Outside corner molding (L - shaped), wood, pre-finished (already painted white so that you don't need to worry about primer and wood grain showing through) - I bought an 8 foot piece at Home Depot

4) Flat black spray paint - I got it at Home Depot while I was there

5) A standard glue - I used Elmers

6) Drill and drill bit the size of the wire/connector coming off of the LED strip

Instructions:

1) Take the back off of the shadowbox and measure the inside dimensions (exactly) up against the front plastic/glass

2) Using a miter box saw, cut the wood corner molding on 45 degree angles to fit around the inside of the frame with one flat surface of the molding against the glass. It needs to be a snug fit, so I advise to cut it a saw blade width longer than you measured. You can always shave off a little more if you have a power miter box saw. Test fit the moldings in place.

3) Remove the corner moldings from the test fit (above) and paint all sides of the molding pieces flat black and let dry. Make sure there is 100% coverage, if not, add another coat (with no drips)
Paint Molding

4) Clean the glass then put the molding back into the frame and glue the outside edge of the corner moldings to the inside frame of the shadow box frame and let dry for at least a half a day.

5) Pick which side of which corner the LED strip wire should go through (top/bottom, right/left). Drill a hole through the corner molding and frame (so that the wire wont be visible inside the frame) at the location you want. Feed the wire/connector through the hole. Test the light to make sure that they work properly. Clean up any saw dust from drilling (I suggest vacuuming the dust so that it doesn't get between the molding and the glass)

6) Add the LED lights around the frame. The sticky backing on the LEDs never seems to hold good enough, so use a very small amount of Elmer's glue or Gorilla hot glue (if you can be quick with it) to help keep them stuck. Cut the LED strip at the last place on the strip that show the scissors icon. The pic below is off another frame that I'm currently working on that I'm adding RGB lights to for displaying some Wonka themed signed stuff and various paraphernalia. Test the lights again to make sure that they still work.
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7) Attach the poster to the back of the shadow box using a method of your choice.

Clean the glass one more time (if needed). Put the back of the backbox back on, mount the frame, and plug it in.

Extra stuff to say - Below is a pic showing some poster board that I also bought at Michaels that reflects the RGB lights differently for each color - pretty cool. I'm going to use that as an edge perimeter that should be a cool effect as the RGB strip fades from one color to the next. I'll post the end result when I eventually get it done. (I know, this thread is about a black light poster frame - I'm going to try something a little more extreme for this next one.)
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