Lately I am trying some of this coffee (it is dark roast):

Origin: Papua New Guinea

Body: Bold
Flavours: Sweet berry fruits, roasted nuts and chocolate sweetness
Brewing: Espresso and Filter (Aeropress + Plunger)

This high altitude, single origin Papua New Guinea Arabica is grown in the highland forests. Family
owned estates grow the original Jamaica Blue Mountain varieties that flourish under the shade of old forests. Blackgold is a beautiful even bean with a silky smooth body, chocolate sweetness, berry fruits and a hint of roasted
nut. A great brew any which way but a filter, stovetop or espresso will produce a stronger brew.

I can not get away from bitter aftertaste, not a chance. Coffee freshness is ok, I have Mazzer Major grinder, dosing 17gr in 18GR VST - tried 34g, 40g, and 25gr extraction. Latest was 17gr to 34gr in 22 seconds (so faster flow) i put a very long flush on my La Scala Butterfly (10 seconds AFTER the boiling water stops going and flow is nice) and still BITTER!

Please advice...

How long ago was the coffee roasted?

It is between 10-15 I think, but can not be so sure although it is well respected roaster here. Problem is same thing happened with 5 day fresh Brazil last time I bought it and I was sure about roasting date. About aftertaste: it is bitter and stays loooong, I am talking 20-30 minutes

did you try a lower extraction temperature? Dark roasts require lower temperatures and I always found the new Guinea/Indonesian beans (those I have used) to have a tendency to powerfull 'dark' taste profiles.

Have you tried the same coffee pulled the same way from that roaster elsewhere to compare? It could just be bad coffee. Bitterness that lingers that long and resonant speaks to either a significant roast defect or bad coffee to start with.

Crema has a taste element that many like but many others perceive as bitter. Try pulling a shot into a standard coffee cup and swirling it around for a full minute before drinking it - that'll mix the crema into the liquid a bit and may help you enjoy it. If that helps but doesn't completely solve the problem for you, stir the shot thoroughly before drinking it and see if that fixes it.

There's a lot of variance in perception of bitterness among humans. Some landmark studies have found that humans have about 2 dozen genes for tasting bitterness, each tuned to respond to different chemicals. Variations in sensitivity to bitterness even affect dietary choice and thus change health outcomes - for example, people who are more sensitive to bitterness eat 25 percent fewer vegetables and (at least in clinical trials) have a higher risk of colon cancer. I don't know if there are any studies looking at different chemical compounds in espresso, but it wouldn't surprise me at all to learn that even different beans and/or different roasts and brew parameters affect the chemical balance (and therefore the taste) of coffee and its components.

Tried lower temperature no help... So I bought fresh 4 day post rost Finca Ucipa - Mexico - and in 2 tries I had very decent cup of espresso So it looks it was coffee after all. Sometimes when we dought ourselves we should just change roaster!

Don't give up yet. Now you know it's not you. With coffees from this part of the world I agree lower temperature is better, down dose to 16g and aim for 32g pour in 30 sec. Also, increase your pre-infusion time if you have that capability. Let us know if that helps. If not...throw it out!

Just add milk

Using a bottomless portafilter? If not, then you really have no idea what's going on with your distribution/extraction. You may be channeling.

Also, you said that you really don't know how old the coffee is. Respectable roasters generally would not omit the roasting date. Get coffee with a firm date on the bag.

I'm also assuming that your equipment is clean (backflushing regularly and detergent flushes at the appropriate intervals).

You may just not like this coffee as espresso or darker roasts in general. Try a different (perhaps lighter roasted) coffee.