muzoon;
Your good questions with decent specific detail is helping you get useful and well-reasoned feedback from Alun and Matt.
Check out the WBA barista competition score/evaluation sheets for more pointers. Review the all judge's and competitor's information.
Your shot time and dose are in the right range. The tamping pressure is a little on the high side, but of course all these factors are inter-related. If you alter tamping pressure a little lighter (say in the 15 to 20 kilos pressure range), then the grind will need to be a little finer to compensate. You will still want the shot time to be about 24 seconds.
Your beans at a day to 2 weeks from roast date is prime freshness standard, but perhaps it is ground too soon? Some cafe's grind coffee to fill the espresso grinder's dosing chamber, then lose cup quality, aroma and crema due to staling in the doser. Brew immediately upon grinding for best results. That means most skilled baristi will use doserless espresso grinders, or keep virtually empty doser hoppers. That means the barista is essentially filling the portafilter by multiple sweeps of the dosing paddle, then levelling and tamping. You do not mention specifics regarding your espresso machine and grinders. If you are using super-automatic machines, then several other factors may also impact results.
I am guessing, without seeing the shots or at least photos, but here are a couple possible factors to spots. Most likely, the spots are loose grinds picked up on the portafilter spout when tamping. KEEP THE WORKSTATION CLEAN AND CLEAR OF SPILLED GRINDS. When tamping, brace the portafilter rim (not spouts) on the edge of the counter with the spout "dangling off the edge". You may prefer to use a computer mouse pad or a similar rubber pad for a tamping surface...keep the pad clean, then you can place the spout on the pad for tamping, without damaging the spout tip. Your roast may have some beans that are too darkly roasted. [You did not mention specifically, but it sounds like your roast is blended green, then roasted. If so, then it should all be relatively similar brown color, without much blackness - according to your description. If you are blending post-roast, then I might have additional suggestions.] The spots MIGHT be caused by portafilter and screens not clean enough, then some of the deposits break loose and enter the brewed coffee. The spots MAY be desirable characteristics of a well pulled shot, like "tiger-striping" in the crema. Photos would help.
The blend you listed adds up to 90%. Perhaps you meant that each ingredient is 1/3 or 33.3%? Perhaps you do not mind posting or PM the remaining details to me?
The green coffee you listed is all described as reasonably high grade, hard bean, washed process arabica.
Modify your blend - ingredients and/or ratios. Increasing the Brasil and decreasing the Guatemalan should increase the sweetness and moderate the acidity, while shifting the color a little darker in tone. Perhaps 40-45% Brasil and 15 or 20% Guatemala will give you an idea of my meaning.
Adding some natural prep or aqua-pulped or semi-washed coffee would boost the body, fruity tones, and yield a darker color tone. Your green suppliers may have appropriate coffee for you to experiment with. You could try similar coffee to your current ingredients. For example a pulped natural Brasil, or a natural Brasil (arabica, not conillon) at say 15 0r 20%, while you keep the Brasil type 2/3 washed component at 15 or 20%. You could try a similar approach with Ethiopian Djimmah, Limmu, or Sidamo/Yrrgacheffe natural (dry preparation) substituting part or all of the washed Sidamo you use now. Be careful to buy exceptioanlly high grade natural prep coffee, or else the physical defects will de-grade the cup quality with off-tastes (muddy, fermented, moldy, earthy, dirty, murky, etc.).
Modify your roast profile. Even without changing the blend ingredients or ratios, a different roast profile can have significan impact on cup characteristics. Reducing the heat application rate will yield a smoother, sweeter espresso. Hard beans like you are using can easily handle a 16 to 22 minute roast duration in a Probat. The trick is to make the roast take longer by adding heat to the beans more slowly, and ending up with a roast that is about the same color as your current roast (not darker). You may need to adjust burner rate, air flow or damping vanes/valves to make the roast profile changes. You might only need to alter the temperature set-points that you use to switch from higher heat rates to lower heat rates. Roasting for espresso is as much about control and finesse of heat application - as it is about skill in green buying and blending.
You may have heard cautions to avoid "baking" the coffee. "Baked" taste occurs when the coffee takes too long to roast because the heat is too little to maintain a continuously increasing bean temperature. So, stretch the roast time longer, but avoid "stalling" the roast.
Alternatively, you might also try a slightly darker coffee roast - a LITTLE further into 2nd crack.
Lots of possibilities...I hope you try a few (scientifically, vary one parameter at a time in order to control your changes) and let us know what results. I would love to hear your feedback after you try a few things.
