epiclevelwarrior
On the Manabase: You obviously want 4 flagstones and the Duals to search them with. That synergy is just too powerful not to run. Bounce lands work well with the GSMs and will help with your colors.
On the Creatures: I would say 8 total creatures that work off the RBs is too many. Cut to 6.
I'm not sure about the marauders. They're an early threat but they don't really help you pull off the RB/fattie setup or stay alive to do so.
On the Other Spells: You want a lower focus on removing time counters and a higher focus on utility effects to make your setup more likely to resolve. You also need a way to beat combo because they go off before you in most situations.
I'd almost tell you to run Blue. Hatching Plans/ Perilous Research would love to be in this deck but 4 colors is probably too much. You desperately need drawing, though, and not the kind that black provides. You want single use cards that dig deep into your library. Perhaps you want to forgo one of the fatties in favour of blue. Only you don't want to lose either one. Nilith is easier to resolve but Gargadon makes the engine work. I wonder if you could get away with it. You'd definitely need a full complement of duals but those, chomatic star, and gemstone mine it might just work...
If it were my deck I would probably go with blue over black.
Sorry if this post is disjointed. I see problems but not many ways to fix them.
You see problems, partly because you're thinking of type 2, rather than block constructed. Blue just doesn't have the splashable spells in TSP block to really work in as a third color or as a fourth. As for the comments on creatures...
If I don't have either a gargadon or a nihilith suspended by turn 2, my game is shot to hell. So yes, 4 of each, because if I have to, I'll mulligan to 5 cards so long as I get one of those two, and the mana to play it. Keldon Marauders is just plain sick with Gargadon. You get 2 to your opponent, probably eat one of his creatures on block or attack, as well as sacrificing him for a time counter. In my book, that reads (1R, Target opponent takes 2 damage, destroy target creature with toughness 3 or less unless opponent pays 3 life, and remove a time counter from Greater Gargadon). Hyper efficient.
I don't have to worry about combo, because the only combo deck worth mentioning in Block constructed is U/G Vesuvan Hermit. And it goes off... one turn before I balance out, usually. Which is just bad for them.
As for all of the time-fixing spells... They let me adjust to drawing RB or a creature too late by fixing my time counters so everything goes off in the right order. Not to mention, Fury Charm will let a gargadon trample through for some serious beatings, or take down a Phyrexian Totem/Prismatic Lens.