Is my Blissey worth to power up? Thanks!

58 candies will not get you over level 30, probably at ~2600cp.
But she is a Dazzling Gleam Blissey (see my other thread: https://pokemongo.gamepress.gg/q-a/best-defensive-blissey), the best gym defender in the meta.
My suggestion: go for it. And walk with another Chansey as your buddy.
My Blissey is also 84iv and P+DG. She already won 1 battle TODAY.

Because of rounding in damage calcs, you may not notice a difference between A=13 or A=15 for IV. If that is your only Blissey, then power it up. I would keep one Chansey as a chansey to keep as a buddy.

Sorry if I'm missing something, but can't you use Blissey as your buddy, and collect Chansey candy?

I have had Dragonite as my buddy (until today - switched to Larvitar!) and I have been collecting Dratini candy.

i don't think you understand IVs. They are just additional to the base stat. so you new base will be 142a/241d/523s compared to 144a/244d/525s. its basically fractions of a percent. 99.9% of the time you will not know the difference.

If you like the moveset, then I'd go ahead and power her up.