Welcome onboard!
I'm by no means an expert but for what I've seen having been used for decades and based on what I've bought/seen been used by fellow builders I'd say any hard enough wood should do. You can look at Janka hardness charts and based on some of them ebony can be three times as hard as walnut but an average value doesn't tell the whole truth. There's harder and softer pieces depending on where the tree has been growing etc. I've got an old parlour guitar from the 30's with a flawless beech fretboard... Also, a lot depends on how you play i.e. do you press the strings on wood or just make them touch the frets, and how often you cut your finger nails. However, black walnut is quite much on the soft side in average so I'm a bit tempted to suggest other options.
I don't know how much is your "expensive" but in my books an AAA Indian rosewood fingerboard blank for a tenner isn't too much. That's what they ask for one at Maderas Barber, Spain. CITES may be an issue, though, so I looked for more local suppliers. https://exoticwoodzone.com/ seem to have no rosewood fretboards but they have some nice woods as "thin dimensional lumber" which is oversize but for $15 including shipping the prices sound reasonable. Another vendor having stuff in stock is https://alliedlutherie.com/collections/fingerboards?sort_by=price-ascending whose prices aren't stellar either.
If you're going to build more than one guitar, ask for 2nd grade. A couple of years ago we placed an order to Maderas Barber and just out of curiosity bought a dozen or so various species B-grade fretboard blanks for €2-3/piece. Exotic ebony, ziricote, santos and madagascar rosewood, granadillo... We could not find any major defects, at least not on the area to be used. Ridiculously beautiful pieces!