Still waiting for you to post a pic.
There should be a valve at the bottom. Make sure it's closed.
Hopefully you can disconnect the tube at the top. If the tube is still flexible, bend it over to let some / most of the fuel drain....into an appropriate container. Next gauge the size of the tube. If above 1/4" ID you may be able to run a gun cleaning rod with a patch down the tube. A 22 caliber gun cleaning rod has a plastic tip on the end with a slot through it like a needle for sewing. The cloth patches slide through the slot and should work well. Measure the inside of the tube by inserting progressively larger drill bits to find the ID. If the ID is larger than 3/8", you may want a larger tip (larger gun caliber) and bigger patches. A Bass Pro Shop or maybe Walmart may have this gun cleaning rod and patches fairly cheap. You want the rod that comes in sections and screws together. The black stuff on the inside of the tube is likely Asphaltene. It should wipe off of the inside of the tube with the gun cleaning patches. DO NOT USE ANY SOLVENT when cleaning the tube, as some will turn the tube from clear to opaque (don't ask me how I know). If you need to wet the patched, use diesel fuel. Some tubes clean easily; some become discolored with age; YMMV.
Ted