Iirc Russia had no Black Sea Fleet in 1878.
I don't know - you may be right.
Russia had two ironclads for coastal defence and four screw corvettes, plus a number of armed merchantmen. However, the Ottomans had more ironclads even without the Mediterranean Fleet.
EDIT: Two OOBs for the Turkish fleet from the Nafziger collection:
1,
2
Biography of the British commander of the Mediterranean squadron at the time, with his plans for intervention:
'To the Right Hon. W. H. Smith, M.P.; Besika Bay, Feb. 8, 1878.
'Mr Layard's private letter of the 6th, showing that the lines of Buyak Tchermedge were to be evacuated, and Constantinople therefore left at the mercy of the Russians, was startling to me, and as his telegrams of the 5th were two days in reaching me, I thought it best to telegraph the news to you immediately. I added that I still thought the Bulair lines might be saved. In saying this I assumed — 1st, That these lines were not included in the neutral zone, or at least that the Turkish troops will not be obliged to evacuate the peninsula; 2d, That the Turks would accept our assistance to defend the lines; 3d, That the Turkish general is not a traitor.
'Given these premises, I think the position might be saved; and, as it is the only one left in Roumelia which we could hold, it may be important to consider the matter. The Russians are said to have 3000 men at Rodosto, sixty miles from Bulair; a force — amount unknown — at Kissen, thirty miles off; and the roads from the north, through Malgara, and generally, are bad. I think, therefore, they could not approach the place under three days, or have a large force there in less than six days.
'In twenty-four hours we could land at Gallipoli a naval brigade of 500 men, and flank the approaches to a certain extent in the ships. This would give the Turks the encouragement and assurance they require, after their recent defeats, to hold the ground for a few days. If orders were sent to the Governor of Malta to co-operate with me, I should send Agincourt, Achilles, and Raleigh to Malta, and they should return in eight days to Gallipoli with 3000 troops. (Distance to Malta, 690 miles; return to Gallipoli, 730 miles.) Steamers should be chartered at Malta, and despatched forty-eight hours after receipt of the telegram, bringing guns, ammunition, biscuit, rum, and cocoa for the troops, and 2000 tons of coals for this squadron. With 3500 English, the ships, and the Turks, I believe we should hold the place for a fortnight against anything the Russians could do.
'By that time — that is, twenty- two days from the receipt of your telegram — you ought to be able to send us the 8000 or 10,000 men that would make this place safe for ever. The first steps will be the most important, and of course the orders must be prompt and decided from home. Troops, ships, and Ambassador would then co-operate. Transport animals and temporary shelter for the troops will be the greatest difficulty, but I believe we can meet them. You may depend I will feed and shelter my own men, and I have great confidence in our contractor, who is an Englishman. I mention this only that you may not suppose such matters have not been considered.'
The fleet ordered to be sent up to Constantinople was Alexandra, Temeraire, Swiftsure, Achilles, Ruby, and Salamis.